Move WAL sequence code into its own file

Move WAL sequence code into its own file

This split exists for most of the other RMGRs, and makes cleaner the
separation between the WAL code, the redo code and the record
description code (already in its own file) when it comes to the sequence
RMGR.  The redo and masking routines are moved to a new file,
sequence_xlog.c.  All the RMGR routines are now located in a new header,
sequence_xlog.h.

This separation is useful for a different patch related to sequences
that I have been working on, where it makes a refactoring of sequence.c
easier if its RMGR routines and its core routines are split.

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Reviewed-by: Kirill Reshke <reshkekirill@gmail.com>
Discussion: https://postgr.es/m/aSfTxIWjiXkTKh1E@paquier.xyz