Speedup of relation deletes during recovery

Hi,

When multiple relations are deleted at the same transaction,
the files of those relations are deleted by one call to smgrdounlinkall(),
which leads to scan whole shared_buffers only one time. OTOH,
during recovery, smgrdounlink() (not smgrdounlinkall()) is called
for each file to delete, which leads to scan shared_buffers multiple times.
Obviously this can cause to increase the WAL replay time very much
especially when shared_buffers is huge.

To alleviate this situation, I'd like to propose to change the recovery
so that it also calls smgrdounlinkall() only one time to delete multiple
relation files. Patch attached. Thought?

Regards,

--
Fujii Masao