[Patch] Make ginInsertCleanup follow vacuum's BufferAccessStrategy

The vacuum callers of ginInsertCleanup weren't able to pass the current BufferAccessStrategy, since the function didn't take a strategy argument. I modified the function to take a strategy, and passed it from the vacuum down to ReadBufferExtended. Other call-sites pass NULL as a strategy, which preserves the current behavior.

I did notice 6c228755add, which uses ReadStream in ginvacuumcleanup()'s full-index scan, but I don't think that will work here: that scan knows the full block range, but the pending-list needs to read each page to get the next block.

Running a quick test (script attached), we do see the strategy changing how GIN treats the buffer cache. I originally noticed this as just an inconsistency, but it may be a good improvement to how GIN vacuum interacts with shared buffers (fewer evictions on other processes).

- Kevin Rocker