Written by Cosmin Apreutesei. Public domain.
Connection pools allow reusing and sharing a limited number of connections
between multiple threads in order to 1) avoid creating too many connections
and 2) avoid the lag of connecting and authenticating every time
a connection is needed.
The pool mechanics is simple (it's just a free list) until the connection
limit is reached and then it gets more complicated because we need to put
the threads on a waiting list and resume them in fifo order and we also
need to remove them from wherever they are on the waiting list on timeout.
This is made easy because we have: 1) a ring buffer that allows removal at
arbitrary positions and 2) sock's interruptible timers.
See the source code for more info.