Fix/test lcbio_MGR maxidle counters

This simplifies (and tests for) the logic of the manager's keeping track
of how many idle connections are allowed per socket. It also removes an
unnecessary variable which can be derived using other fielss.

Change-Id: I3f6ace4c581129de2a4b24562c203c6c37a36292
Reviewed-on: http://review.couchbase.org/43721
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>