Streamlining wal_level and monitoring

The wal_level setting has always been a bit hard to understand for many people. Many were struggling with the difference between the archive and hot_standby settings. To remove this confusion altogether, both settings have been replaced with the easier-to-understand replica setting, which does the same as hot_standby.

In addition to that, the monitoring of replicated setups has been simplified. Prior to 9.6, there was only the pg_stat_replication view, which could be queried on the master to supervise the flow of data to the slave. Now it is also possible to monitor the flow of data on the slaves, by consulting the pg_stat_wal_receiver function. It is basically the slave-side mirror of the pg_stat_replication function and helps to determine the state of replication.