dhcpv6 server assigns reservations from the pools even if out of pool flag is set

this was discovered while implementing #1405 (closed) and seeing the different behavior from v4 in previous written UT:

TEST_F(DORATest, reservationModeOutOfPool)

TEST_F(DORATest, reservationIgnoredInOutOfPoolMode)

in v4, before dynamic allocation (from pool), all reservations from the pools are removed.

in v6, because the retrieval of the reservations is done once for all IANAs, there is no filtering, and this must be done later, just before the dynamic allocation from the pools. at this stage, the lease type is also available, so proper filtering from the pool is optimal.