RelationalSnapshotChangeEventSource should accept a RelationalDatabaseSchema

As PostgresSnapshotChangeEventSource extends RelationalSnapshotChangeEventSource which uses HistorizedRelationalDatabaseSchema and PostgreSQL doesn't support HistorizedRelationalDatabaseSchema so any schema-specific operation can't be performed in this level. 

As suggested by ccranfor@redhat.com there might be a need for HistorizedRelationalSnapshotChangeEventSource implementation that overrides whatever is needed for the historized implementation.