Create new errcode for recovery conflict caused by db drop on master.

Create new errcode for recovery conflict caused by db drop on master.
Previously reported as ERRCODE_ADMIN_SHUTDOWN, this case is now
reported as ERRCODE_DATABASE_DROPPED. No message text change.
Unlikely to happen on most servers, so low impact change to allow
session poolers to correctly handle this situation.

Tatsuo Ishii and Simon Riggs