enable conditional failover for read/read_only database connections

[alfredodeza]

The read and read_only callables would be taken care of by the TransactionalHook in pecan. But these callables need to be a bit smart to use configuration to determine what is the read or read_only database and ignore according to a promotion of a failed database (for example if the master fails and a read_only is promoted, use the read_only db as the new master).