Persist design docs into vbucket 0xffff.

The vbucket for design docs is special in that its not registered in
livebucket.vbuckets array.  By storing ddocs in a vbucket, we reuse
the existing CAS assignment, changes stream tracking, flushing,
compaction, passivation and other features provided by vbuckets.
REST api to report on the vbucket for design docs.