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

Create new errcode for recovery conflict caused by db drop on 
Previously reported as ERRCODE_ADMIN_SHUTDOWN, this case is now
reported as ERRCODE_T_R_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, edits by me, review by Robert Haas