Support TCP Connection Reuse?

[woluohenga]

Now meta protocol proxy use tcp conn pool, it while cause many connections when high concurrent requests。
So can we support tcp connection reuse?

[zhaohuabing]

To make it clear: MetaProtcolProxy now has a connection pool for each upstream cluster and reuses idle connections in the pool when proxying the requests to upstream clusters. What this PR does is support multiplexing, thus, allow sending multiple requests to the same upstream cluster in a single connection to that cluster.