partially revert 98bd540c (adding more fields to backend valiation)

Turns out some of the fields (databaseId/roleId) are not resolved
yet when calling backend_update_info(), so the check was always
failing (so all backends were invalid all the time, and all the
counts were 0 all the time).

Check only pid/backendId, remove the other fields - this seems to
be working just fine even for crashed backends (even pid alone
would be probably OK).
added a section about limitatations into the README