Remove WalCompressionMethod in favor of pg_compress_algorithm

Remove WalCompressionMethod in favor of pg_compress_algorithm

The same structure, with the same set of elements (for none, lz4, gzip
and zstd), exists in compression.h, so let's make use of the centralized
version instead of duplicating things.  Some of the variables used
previously for WalCompressionMethod are renamed to stick better with the
new structure and routine names.

WalCompressionMethod was leading to some confusion in walmethods.c, as
it was sometimes used to refer to some data unrelated to WAL.

Reported-by: Robert Haas
Author: Michael Paquier
Reviewed-by: Robert Haas, Georgios Kokolatos
Discussion: https://postgr.es/m/YlPQGNAAa04raObK@paquier.xyz