Sweep idle UDP connections

[redneck-f25]

When sending many UDP datagrams from many clients (every 10 seconds from 3500 clients), I ran out of available ephemeral ports.

• Add per-connection activity tracking (lastUsed)
• Replace read-deadline based cleanup with activity-based idle timeout
  (connections are now kept alive as long as there is traffic in either direction)
• Implement background sweeper with configurable timeout (env:UDP_IDLE_TIMEOUT)
• Move closeAll() from udpConns into Tunnel (closeAllUDPConnections) to reflect ownership
• Concerns a long living TODO

EDIT 2026-07-01

Today I used the patched version in the field and provide some real-world numbers for comparison.

Every 5 seconds, the TCP (WebSocket) connections established by Chisel clients to the Chisel Server (via a TLS-Offloader) and UDP sockets from the Chisel server to the receiving service were counted.

The average number of UDP sockets per TCP connection has been reduced from 4.5 to 1.5. Furthermore, the rate remains stable during the measurement period, and the resources required are therefore predictable.

| before patch         | WS in | UDP out | after patch          | WS in | TCP out| diff | diff   |
|----------------------|-------|---------|----------------------|-------|--------|------|--------|
| 2026-06-30T13:00:00Z |  3260 |   14672 | 2026-07-01T13:00:01Z |  3295 |   4953 |   35 |  -9719 |
| 2026-06-30T13:00:05Z |  3261 |   16138 | 2026-07-01T13:00:07Z |  3292 |   4972 |   31 | -11166 |
| 2026-06-30T13:00:11Z |  3261 |   10884 | 2026-07-01T13:00:12Z |  3288 |   5003 |   27 |  -5881 |
| 2026-06-30T13:00:16Z |  3262 |   12594 | 2026-07-01T13:00:17Z |  3287 |   5090 |   25 |  -7504 |
| 2026-06-30T13:00:21Z |  3259 |   14138 | 2026-07-01T13:00:22Z |  3286 |   4990 |   27 |  -9148 |
| 2026-06-30T13:00:27Z |  3257 |   15581 | 2026-07-01T13:00:28Z |  3284 |   4973 |   27 | -10608 |
| 2026-06-30T13:00:32Z |  3259 |   17362 | 2026-07-01T13:00:33Z |  3284 |   4978 |   25 | -12384 |
| 2026-06-30T13:00:37Z |  3261 |   18837 | 2026-07-01T13:00:38Z |  3283 |   5051 |   22 | -13786 |
| 2026-06-30T13:00:43Z |  3260 |   20616 | 2026-07-01T13:00:43Z |  3284 |   4934 |   24 | -15682 |
| 2026-06-30T13:00:48Z |  3263 |   22087 | 2026-07-01T13:00:49Z |  3284 |   4978 |   21 | -17109 |
| 2026-06-30T13:00:54Z |  3263 |    9757 | 2026-07-01T13:00:54Z |  3283 |   4962 |   20 |  -4795 |
| 2026-06-30T13:00:59Z |  3263 |   11474 | 2026-07-01T13:00:59Z |  3282 |   5023 |   19 |  -6451 |
| 2026-06-30T13:01:04Z |  3261 |   12966 | 2026-07-01T13:01:04Z |  3280 |   4962 |   19 |  -8004 |
| 2026-06-30T13:01:10Z |  3257 |   14464 | 2026-07-01T13:01:10Z |  3280 |   4989 |   23 |  -9475 |
| 2026-06-30T13:01:15Z |  3260 |   16226 | 2026-07-01T13:01:15Z |  3276 |   5013 |   16 | -11213 |
| 2026-06-30T13:01:20Z |  3261 |   10850 | 2026-07-01T13:01:20Z |  3276 |   4994 |   15 |  -5856 |
| 2026-06-30T13:01:26Z |  3256 |   12316 | 2026-07-01T13:01:25Z |  3274 |   4989 |   18 |  -7327 |
| 2026-06-30T13:01:31Z |  3258 |   14087 | 2026-07-01T13:01:31Z |  3273 |   4988 |   15 |  -9099 |
| 2026-06-30T13:01:36Z |  3252 |   15545 | 2026-07-01T13:01:36Z |  3270 |   5018 |   18 | -10527 |
| 2026-06-30T13:01:42Z |  3249 |   17084 | 2026-07-01T13:01:41Z |  3268 |   4947 |   19 | -12137 |
| 2026-06-30T13:01:47Z |  3250 |   18807 | 2026-07-01T13:01:46Z |  3270 |   4985 |   20 | -13822 |
| 2026-06-30T13:01:52Z |  3245 |   20308 | 2026-07-01T13:01:52Z |  3269 |   4908 |   24 | -15400 |
| 2026-06-30T13:01:58Z |  3250 |   22052 | 2026-07-01T13:01:57Z |  3268 |   4992 |   18 | -17060 |
| 2026-06-30T13:02:03Z |  3247 |   11793 | 2026-07-01T13:02:02Z |  3268 |   4952 |   21 |  -6841 |
| 2026-06-30T13:02:08Z |  3250 |   11143 | 2026-07-01T13:02:07Z |  3270 |   4972 |   20 |  -6171 |
| 2026-06-30T13:02:14Z |  3252 |   12859 | 2026-07-01T13:02:13Z |  3268 |   4926 |   16 |  -7933 |
| 2026-06-30T13:02:19Z |  3253 |   14390 | 2026-07-01T13:02:18Z |  3264 |   5021 |   11 |  -9369 |
| 2026-06-30T13:02:24Z |  3257 |    9104 | 2026-07-01T13:02:23Z |  3266 |   4936 |    9 |  -4168 |
| 2026-06-30T13:02:30Z |  3257 |   10649 | 2026-07-01T13:02:28Z |  3261 |   4991 |    4 |  -5658 |
| 2026-06-30T13:02:35Z |  3256 |   12352 | 2026-07-01T13:02:33Z |  3265 |   4911 |    9 |  -7441 |
| 2026-06-30T13:02:40Z |  3247 |   13864 | 2026-07-01T13:02:39Z |  3267 |   4980 |   20 |  -8884 |
|----------------------|-------|---------|----------------------|-------|--------|------|--------|
| min     (2.8 in/out) |  3245 |    9104 |         (1.5 in/out) |  3261 |   4908 |   +4 |  -4168 |
| max     (6.8 in/out) |  3263 |   22087 |         (1.5 in/out) |  3295 |   5090 |  +35 | -17109 |
| avg     (4.5 in/out) |  3256 |   14677 |         (1.5 in/out) |  3276 |   4980 |  +20 |  -9697 |