Rename config types for clarity.

This changeset renames the DefaultConfig in CouchbaseConfig and
the CacheConfig into MemcacheConfig to more accurately describe
the type of buckets they are referring to.

Change-Id: Icfa43b63595a362d14c1b0938094fc1d4fcc56b3
Reviewed-on: http://review.couchbase.org/30373
Reviewed-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>