Don't send unnecessary config updates.

The memcached service should keep track of the last revId that it sent to a client who is connected, and avoid dispatching an identical config to the client during a NMV. Currently, when pipelining operations, a client may receive hundreds of NMV responses when only a single one is necessary. This won't prevent multiple NMV's across nodes, but will prevent the bulk of the spam.

Note: An explicit request for the config via CCCP_GET_VBUCKET_CONFIG should always return the data regardless of revId info.