Scan the buffer pool just once, not once per fork, during relation drop.

Scan the buffer pool just once, not once per fork, during relation drop.

This provides a speedup of about 4X when NBuffers is large enough.
There is also a useful reduction in sinval traffic, since we
only do CacheInvalidateSmgr() once not once per fork.

Simon Riggs, reviewed and somewhat revised by Tom Lane