Return to IDLE properly when connection lulls

Commit 96aaf3 fixed a bug with regard to IDLE status responses being
improperly reported to ClientSession.  As it turned out, that fix
introduced a bug by not properly decrementing outstanding_cmds, which
left the ClientConnection in a state where it would never enter IDLE
thereafter.

The problem is a change I pondered at the time but didn't commit due
to that fix being so close to release time: not reporting the original
IDLE command to ClientSession either.  That in itself is harmless, but
not doing that also avoids incrementing outstanding_cmds for that
command, meaning when the IDLE completes that value returns to zero
and the ClientConnection is in an appropriate state to return to IDLE
when ready.