RPKI: Fix unnecessary reconnection on reconfiguration

Compare the new timing parameters with the old configuration, not with
the temporary state of the current connection.

The timing values in struct rpki_cache is updated by a version 1 End Of
Data PDU, unless this behavior is suppressed by the configuration
explicitly by the "keep" keyword. Consequently, every reconfiguration
of BIRD triggers a reconnection even if it is not necessary.