localbuf: Add Valgrind buffer access instrumentation

localbuf: Add Valgrind buffer access instrumentation

This mirrors 1e0dfd166b3 (+ 46ef520b9566), for temporary table buffers. This
is mainly interesting right now because the AIO work currently triggers
spurious valgrind errors, and the fix for that is cleaner if temp buffers
behave the same as shared buffers.

This requires one change beyond the annotations themselves, namely to pin
local buffers while writing them out in FlushRelationBuffers().

Reviewed-by: Noah Misch <noah@leadboat.com>
Co-authored-by: Noah Misch <noah@leadboat.com>
Discussion: https://postgr.es/m/3pd4322mogfmdd5nln3zphdwhtmq3rzdldqjwb2sfqzcgs22lf@ok2gletdaoe6