pgsql: Move the server's backup manifest code to a separate file.

Move the server's backup manifest code to a separate file.

basebackup.c is already a pretty big and complicated file, so it
makes more sense to keep the backup manifest support routines
in a separate file, for clarity and ease of maintenance.

Discussion: http://postgr.es/m/CA+TgmoavRak5OdP76P8eJExDYhPEKWjMb0sxW7dF01dWFgE=uA@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/079ac29d4dafe581748ceca523aa90c8ce8b035b

Modified Files
--------------
src/backend/replication/Makefile          |   1 +
src/backend/replication/backup_manifest.c | 375 ++++++++++++++++++++++++++++
src/backend/replication/basebackup.c      | 391 +-----------------------------
src/include/replication/backup_manifest.h |  51 ++++
4 files changed, 429 insertions(+), 389 deletions(-)