Add PostgreSQL replication lag to collectd

Create a stored procedure to calculate the number of bytes secondaries
are lagging from the primary. This is only tracked on the primary so it
is only added to the collectd query list there.
Use md5 for postgres monitoring auth

'trust' over tcp is a headache and doesn't seem to work with pgsql.
Use monitoring password
GRANT monitoring SELECT on stagecraft

so that it can get all the data
Fix monitoring grant