Rename XLogData protocol message to WALData

Rename XLogData protocol message to WALData

This name is only used as documentation, and using this name is
consistent with its byte being a 'w'.  Renaming it would also make the
use of a symbolic name based on the word "WAL" rather than the obsolete
"XLog" term more consistent, per future commits along the lines of
37c7a7eeb6d1, 4a68d5008869, f4b54e1ed985.

Discussion: https://postgr.es/m/aIECfYfevCUpenBT@nathan