Patch "d130255e" disabled encoding of SYNC in xrow_encode_header()
to avoid writing SYNC value into the disk. Unfortunally, this change
has also broken all other users of xrow_encode_header(), such as
replication (JOIN/SUBSCRIBE requests).

Uncomment row->sync encoding in xrow_header_encode() and
hack xlog_write_row() to avoid writing SYNC into WAL.

Closes #2334