MEDIUM: check: implement check-reuse-pool

MEDIUM: check: implement check-reuse-pool

Implement the possibility to reuse idle connections when performing
server checks. This is done thanks to the recently introduced functions
be_calculate_conn_hash() and be_reuse_connection().

One side effect of this change is that be_calculate_conn_hash() can now
be called with a NULL stream instance. As such, part of the functions
are adjusted accordingly.

Note that to simplify configuration, connection reuse is not performed
if any specific check connection parameters are defined on the server
line or via the tcp-check connect rule. This is performed via newly
defined tcpcheck_use_nondefault_connect().