Arrange to emit a description of the current XLOG record as error context

Arrange to emit a description of the current XLOG record as error context
when an error occurs during xlog replay.  Also, replace the former risky
'write into a fixed-size buffer with no overflow detection' API for XLOG
record description routines; use an expansible StringInfo instead.  (The
latter accounts for most of the patch bulk.)

Qingqing Zhou