Add WAL logging for CREATE/DROP DATABASE and CREATE/DROP TABLESPACE.

Add WAL logging for CREATE/DROP DATABASE and CREATE/DROP TABLESPACE.
Fix TablespaceCreateDbspace() to be able to create a dummy directory
in place of a dropped tablespace's symlink.  This eliminates the open
problem of a PANIC during WAL replay when a replayed action attempts
to touch a file in a since-deleted tablespace.  It also makes for a
significant improvement in the usability of PITR replay.