Added checks for `--delta` and `--force` restore options to ensure that the destination is a valid $PGDATA directory. pgBackRest will check for the presence of `PG_VERSION` or `backup.manifest` (left over from an aborted restore). If neither is found then `--delta` and `--force` will be disabled but the restore will proceed unless there are files in the $PGDATA directory (or any tablespace directories) in which case the operation will be aborted.
* Major refactoring of the protocol layer to support this work.
* Fixed protocol issue that was preventing ssh errors (especially connect) from being logged.
Replaced IPC::System::Simple and Net::OpenSSH with IPC::Open3 to eliminate CPAN dependency for multiple distros. Using open3 will also be used for local processes so it make sense to switch now.
* 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.
