update to "couchbase" from "membase" in gerrit mirroring and manifests

One of the key components of Couchbase is still only at github.com/membase and not at github.com/couchbase. I think it's okay to mirror to both locations (not that there's an advantage), but for sure it should be at couchbase and the manifest for Couchbase Server releases should be pointing to Couchbase.

I believe the steps here are as follows:

• Set up a github.com/couchbase/memcached project (I've done that)
• Update gerrit's commit hook to update that repository
• Change the manifests to start using that repository

Assigning this to build as a component, as gerrit is handled by the build team. Then I'm guessing it'll need to be handed over to Trond or another developer to do the manifest change once gerrit is up to date.

Since memcached is slow changing now, perhaps the third item can be done earlier.