Allow I/O reliability checks using 16-bit checksums

Checksums are set immediately prior to flush out of shared buffers
and checked when pages are read in again. Hint bit setting will
require full page write when block is dirtied, which causes various
infrastructure changes. Extensive comments, docs and README.

WARNING message thrown if checksum fails on non-all zeroes page;
ERROR thrown but can be disabled with ignore_checksum_failure = on.

Feature enabled by an initdb option, since transition from option off
to option on is long and complex and has not yet been implemented.
Default is not to use checksums.

Checksum used is WAL CRC-32 truncated to 16-bits.

Simon Riggs, Jeff Davis, Greg Smith
Wide input and assistance from many community members. Thank you.

src/backend/access/hash/hash.c

@@ -285,11 +285,9 @@ hashgettuple(PG_FUNCTION_ARGS)
 			ItemIdMarkDead(PageGetItemId(page, offnum));
 
 			/*
-			 * Since this can be redone later if needed, it's treated the same
-			 * as a commit-hint-bit status update for heap tuples: we mark the
-			 * buffer dirty but don't make a WAL log entry.
+			 * Since this can be redone later if needed, mark as a hint.
 			 */
-			SetBufferCommitInfoNeedsSave(buf);
+			MarkBufferDirtyHint(buf);
 		}
 
 		/*