dsn: preparse logging level

This allows us to set the logging level early on in the library, rather
than deferring it to later invocations of lcb_cntl()

Change-Id: I93fd4c46c14d6d7de4267359289cd328cd4f1b85
Reviewed-on: http://review.couchbase.org/38372
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>