Fix socket accounting for TCP accept() (#854)

Fix socket accounting for TCP accept() (#854)

The incoming_sockets_accepted counter is used by Comm I/O modules to
count the number of successfully accept()ed FDs during a single
select(2) (or equivalent) scan of ready descriptors. For TCP
connections, the official code incremented that counter in the async
TcpAcceptor callbacks. Those callbacks run outside the counting Comm I/O
context, rendering counter increments useless and resulting in
under-reporting of actual acceptance activity.

Also, some successfully accepted TCP connections fire no callbacks, so
it is conceptually wrong to count accepted sockets in callbacks,
regardless of the timing problems.