More separation of the protocol and remote layers than was done in issue #106.
Settings are passed to the remote via command-line parameters rather than in the protocol.
* Includes updating the manifest to format 4. It turns out the manifest and .info files were not very good for providing information. A format update was required anyway so worked through the backlog of changes that would require a format change.
* Multiple database versions are now supported in the archive. Does't actually work yet but the structure should be good.
* Tests use more constants now that test logs can catch name regressions.
Moved Remote code from pg_backrest.pl to Config.pm.
Added version specific code to regression tests and Db.pm.
archive-push checks for duplicate WAL in the archive.
archive-push reads the db sys id to match up WAL to the correct archive.
