Check that sufficient walsenders will be available on the promotion
candidate, and if replication slots are in use check if enough of
those will be available.
Note these checks can't guarantee that the walsenders/slots will
be available at the appropriate points during the switchover process,
but do ensure that existing configuration problems will be caught.
Implements GitHub #371.
In particular, if running "repmgr cluster show" against a database
without the repmgr metadata, showing the error (rather than just
"no records found" etc.) will provide some clues about the problem.
Previously, if an issue was encountered with the old primary, but user
provided -F/--force to have repmgr promote the standby anyway, repmgr
would exit with the log message "STANDBY SWITCHOVER is complete"
and exit code 0 (SUCCESS).
To better report this partial completion, repmgr will now emit the message
"STANDBY SWITCHOVER has completed with issues" (and a HINT to check preceding
log messages) and new exit code 22 (ERR_SWITCHOVER_INCOMPLETE).
