This patch fixes a problem in the closing of timed-out connections to
targets in back-asyncmeta. If a a_metaconn is still in use for another
target, the connections to other targets might not be reset while the mc
is still active, even if these targets are no longer in use. This patch
introduces counters for pending operations per a_metasingleconn, and
resets a metasingleconn if it has timed out and has no pending operations.