Fix stale shards cache
Fix stale shards cache

There's a race condition in mem3_shards that can result in having shards
in the cache for a database that's been deleted. This results in a
confused cluster that thinks a database exists until you attempt to open
it.

The fix is to ignore any cache insert requests that come from an older
version of the dbs db than mem3_shards cache knows about.

Big thanks to @jdoane for the identification and original patch.

COUCHDB-3376

fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb