MB-7535 Promote some logging from debug to info level

For all log messages related to monitoring index updates,
promote their level from debug to info. Also, everytime
an indexed sequence is not high enough to unblock a client,
log a message mentioning this information.

Change-Id: I4cd8f98d585dd67909e680a210a1d9e15b86517c
Reviewed-on: http://review.couchbase.org/24020
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>