Fix cleanup type issues with non-sqlite backends
    
    We store the data as strings, so when telling SQL to compare, we should be
    sending strings to compare against.
    SQLite does not actually check types, so doesn't throw this error, but for
    example in Postgres, the following error is being recorded:
    ProgrammingError: (psycopg2.ProgrammingError) operator does not exist: text <= timestamp without time zone
    
    Merges: #120
    Signed-off-by: Patrick Uiterwijk <puiterwijk@redhat.com>
    Reviewed-by: Pierre-Yves Chibon <pingou@pingoured.fr>
    Reviewed-by: Howard Johnson <merlin@merlinthp.org>