Fix unaligned memory access in xlog parsing due to replication origin patch.

Fix unaligned memory access in xlog parsing due to replication origin patch.

ParseCommitRecord() accessed xl_xact_origin directly. But the chunks in
the commit record's data only have 4 byte alignment, whereas
xl_xact_origin's members require 8 byte alignment on some
platforms. Update comments to make not of that and copy the record to
stack local storage before reading.

With help from Stefan Kaltenbrunner in pinning down the buildfarm and
verifying the fix.