[check_postgres] check_sequence for more than one database

[Christoph Zwerschke]

I found check_sequence does not work correctly when used with more than
one database (--dbname=db1,db2).

It seems to check sequences from one database in the other database,
resulting in errors like: "relation myschema.mysequence does not exist".

As a fix, the run_command($SQL) in the loop running over the databases
must be probably called with the corresponding dbnumber.

-- Christoph