Telnet: buffered output for AsyncTcp server

[mcspr]

• buffer outgoing data in the internal linked list after client would not accept any more data
• schedule telnet client destruction on loop after disconnection routine, see Fix for tcp_err() callbacks and other issues me-no-dev/ESPAsyncTCP#115

This is something I had noticed with Sonoff Basic board running lwip2 low-memory
scheduling is +400B
buffering is +1.4KB

Why not AsyncTCPbuffer? -1KB code size and slightly less memory allocations.
And technically, nothing stops us from using this class for something else.

[mcspr]

It would also make sense to always buffer / add callback (#1909) for shared data (debug, ws announce). But it feels like it would be much easier to implement this with another class that only does that specific thing on a list of clients, pushing chunks of data in a queue until every client has it (...onAck for each client should count written data)

Debugging itself needs a queue of sorts to help with that, but that is a whole other issue.
edit:Linux printk could be used as a reference for some of the parts.
edit:Rsyslog error messaging (LogMsg func), internal message queue (submitMsg2), etc.
edit:Rsyslog queue (internal module)
edit:Syslog-ng's queueing should be explored too
edit2 ... formatting

Also a honorable mention of AsyncPrinter.cpp, but technically we don't need (or want, because of delay() usage) Stream API.