Allow dropping nodes in all cases, create a primary from a non-candidate

[DimCitus]

At the moment we see in https://github.com/citusdata/pg_auto_failover/blob/master/src/monitor/node_active_protocol.c#L1044 that we refuse to remove the primary node in the monitor when we don't have a candidate for promotion. It would be better to allow for that node removal, and this requires the ability to create a primary node from existing secondary node(s).

Also, when adding a primary from secondary nodes is possible, this would allow pg_auto_failover setups with only non-candidate nodes.

In passing, we might want to improve the REPORT_LSN state to allow for the secondary node to be running, probably with fetching the current LSN and marking it as the target LSN in recovery mode while in the REPORT_LSN state.

[DimCitus]

Fixed in #661.