[5247] Corrected issues with assigned-<lease/nas/pd> stats

[5247] Corrected issues with assigned-<lease/nas/pd> stats

doc/guide/dhcp4-srv.xml
doc/guide/dhcp6-srv.xml
    Added entries for reclaimed-leases

src/lib/dhcpsrv/alloc_engine.cc

    AllocEngine::reuseExpiredLease(Lease6Ptr&...) - increment
    assigned-<nas/pds> for real allocations

    AllocEngine::extendLease6() - increment assigned-<nas/pds>
    for real allocations if the lease expired

    AllocEngine::renewLease4(const Lease4Ptr&...)
    - set lease state to STATE_DEFAULT for real allocations
    - increment assigned-leases if lease is expired or reclaimed
      for real allocations

    AllocEngine::reuseExpiredLease4(Lease4Ptr&...)
    - increment assigned-leases for real allocations

src/lib/dhcpsrv/cfg_subnets4.cc
    CfgSubnets4::removeStatistics() - added "reclaimed-leases"

src/lib/dhcpsrv/cfg_subnets6.cc
    CfgSubnets4::removeStatistics() - added "reclaimed-leases"

src/lib/dhcpsrv/lease_mgr.cc
    LeaseMgr::recountLeaseStats4()
    LeaseMgr::recountLeaseStats6()
    - added handling of "reclaimed-leases"
    - fixed name of "reclaimed-declined-addresses"

src/lib/dhcpsrv/tests/alloc_engine4_unittest.cc
    TEST_F(AllocEngine4Test, simpleRenew4)  - new test to verify stats
    on a normal renew, non-expired
    Added EXPECT_TRUE around calls to testStatistics for invocation line numbers
    Added stat checks to several tests

src/lib/dhcpsrv/tests/alloc_engine6_unittest.cc
    Added EXPECT_TRUE around calls to testStatistics for invocation line numbers
    Added stat checks to several tests

src/lib/dhcpsrv/tests/alloc_engine_expiration_unittest.cc
    Added checks of assigned stats

src/lib/dhcpsrv/tests/alloc_engine_utils.cc
    AllocEngine6Test::initSubnet() - removed artificial stat values

    NakedAllocEngine::addHost() - new method to add a host to the
    current configuration, rather than use staging/commit as the latter