Fix FATAL ServiceRep::putConnection exception: theBusyConns > 0 (#939)

Fix FATAL ServiceRep::putConnection exception: theBusyConns > 0 (#939)

    FATAL: check failed: theBusyConns > 0
        exception location: ServiceRep.cc(163) putConnection

Since master/v6 commit 2b6b1bc, a timeout on a ready-to-shovel
Squid-service ICAP connection was decrementing theBusyConns level one
extra time because Adaptation::Icap::Xaction::noteCommTimedout() started
calling both noteConnectionFailed() and closeConnection(). Depending on
the actual theBusyConns level, the extra decrement could result in FATAL
errors later, when putConnection() was called (for a different ICAP
transaction) with zero theBusyConns in an exception-unprotected context.

Throughout these changes, Xaction still counts the above timeouts as a
service failure. That is done by calling ServiceRep::noteFailure() from
Xaction::callException(), including in timeout cases described above.