Add a simple implementation of max connections

max limits the sum of active connections and listeners.
The implementation assumes that max is greater than acceptor_pool_size,
and always maintains the acceptor_pool at acceptor_pool_size as long
as the max is not exceeded.

If max is less than acceptor_pool_size, acceptor_pool_size is the
effective max.
Add eunit tests for the max implementation.
Move the mochiweb_socket_server_tests to the test directory.
Avoid sleeping in the unittests.
Fix the computation of the active_sockets number.

If we added a new socket in the recycle_acceptor, we should not decrement the
number of active sockets.