Rename backup_compression.{c,h} to compression.{c,h}

Rename backup_compression.{c,h} to compression.{c,h}

Compression option handling (level, algorithm or even workers) can be
used across several parts of the system and not only base backups.
Structures, objects and routines are renamed in consequence, to remove
the concept of base backups from this part of the code making this
change straight-forward.

pg_receivewal, that has gained support for LZ4 since babbbb5, will make
use of this infrastructure for its set of compression options, bringing
more consistency with pg_basebackup.  This cleanup needs to be done
before releasing a beta of 15.  pg_dump is a potential future target, as
well, and adding more compression options to it may happen in 16~.

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