CCBC-482: Don't throttle based on file-based initial config

This quickens recovery time based on a stale config.

This commit also changes the behavior so that the last_refresh timestamp
is always updated when a valid new config is received (which is not
file-based)

Change-Id: I1fa3376ec002d2d05ea559a68262b01199629a68
Reviewed-on: http://review.couchbase.org/39973
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>