MB-100: implemented ns_config:run_txn

Which implements optimistic transactions on top of cas_config. And
thus avoids running arbitrary and potentially dangerous code inside
ns_server.

Change-Id: I59bf6a192baad4a6c6f370a381931111df81ec7c
Reviewed-on: http://review.couchbase.org/29865
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Merge branch 'for-rackaware' (early part)

* 'for-rackaware' (early part):
  MB-100: implemented ns_config:run_txn

This is trivial merge

Change-Id: I457a2145ca2df50daf03f25891aadb567a804bc2