Close config connection before trying next node

It will fix asserts in case of the config node becomes unresponsive,
and the threshold controlled by LCB_CNTL_CONFERRTHRESH and  lcb_cntl

Change-Id: If15cdc59278054798b778b908fb8ab422d8eebf5
Reviewed-on: http://review.couchbase.org/28873
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>