Initial switchover implementation

The repmgr3 implementation required the promotion candidate (standby)
to directly work with the demotion candidate's data directory,
directly execute server control commands etc.

Here we delegated a lot more of that work to the repmgr on the
demotion candidate, which reduces the amount of back-and-forth
over SSH and generally makes things cleaner and smoother.

In particular the repmgr on the demotion candidate will carry
out a thorough check that the node is shut down and report
the last checkpoint LSN to the promotion candidate; this
can then be used to determine whether pg_rewind needs to be
executed on the demoted primary before reintegrating it back
into the cluster (todo).

Also implement "--dry-run" for this action, which will sanity-check the
nodes as far as possible without executing the switchover.

Additionally some of the new repmgr node commands (or command options)
introduced for this can be also executed by the user to obtain
additional information about the status of each node.

controldata.h

@@ -24,4 +24,7 @@ get_db_state(char *data_directory);
 extern const char *
 describe_db_state(DBState state);
 
+extern XLogRecPtr
+get_latest_checkpoint_location(char *data_directory);
+
 #endif /* _CONTROLDATA_H_ */