Move WAL/RMGR sequence code into its own file and header

Hi all,

Something that has been bugging me in the sequence code, while doing
some recent work (cough), is the fact that it is possible to cleanly
split the RMGR sequence code from the main sequence.c.

Please find attached a patch doing that, cleaning a bit sequence.c by
removing some of the WAL bits in it, as of:
- Addition of a new file sequence_xlog.c.
- Addition of a new header, sequence_xlog.h.

SEQ_MAGIC and sequence_magic need to be moved from sequence.c to the
new sequence_xlog.h, separating the WAL insert code with the backend
redo parts of it, which is more consistent with other RMGRs.

I have been looking at some other parts of the backend (like the
tablespace part), but these don't really bring a clear gain like the
change in the attached does.

Thanks,
--
Michael