slave_connections_needed_for_purge prevents independent machine from purging binary logs

With variable slave_connections_needed_for_purge, a primary machine needs to check replicas before purging binary logs. If a replica is not there and the machine is a standalone machine, it just does not purge the binary logs and ignores log_expire_days variable and binary logs fills the disk space.
Binary log purge is not happening due to the default behavior of `slave_connections_needed_for_purge` to `1` on a standalone server.

The default value for slave_connections_needed_for_purge should be changed to 0 (i.e. assume the server will be standalone); then if a slave connects, log a warning so users know to change the value.

For reference, the original solution proposed when the JIRA was created
_If the MariaDB is able to determine whether we are a standalone installation and if so set default value slave_connections_needed_for_purge to 0.
The default of 1 is not appropriate unless we are in fact part of a replica group._