Verify document expiration behavior

The DCP documentation talks about possible unexpected behavior around document expiration. Test this and make sure the DCP client is propagating the right kind of event to the user.

https://github.com/couchbase/kv_engine/blob/master/docs/dcp/documentation/expiry-opcode-output.md#why-am-i-getting-mutations-instead-of-deletionsexpirations-on-my-expired-documents