[5252] Corrected lease state and assigned stat when reusing expired-reclaimed via...

[5252] Corrected lease state and assigned stat when reusing expired-reclaimed via V6 request

src/lib/dhcpsrv/alloc_engine.cc
    AllocEngine::updateLeaseData() - added logic to set lease state
    to default and increment assigned stat if the lease is expired-reclaimed

src/lib/dhcpsrv/tests/alloc_engine6_unittest.cc
    TEST_F(AllocEngine6Test, requestReuseDeclinedLease6Stats) - new test
    to verify the behavior