[5585] Initial LeaseQueryStats, LeaseMgr, and MemfileLeaseMgr extensions

[5585] Initial LeaseQueryStats, LeaseMgr, and MemfileLeaseMgr extensions

    MemfileLeaseMgr unit tests fail for subnet and range query variants, not yet
    implemented

src/lib/dhcpsrv/lease_mgr.*
    LeaseStatsQuery
    - Added SelectMode enum typedef
    - Added instance members: first_subnet_id_, last_subnet_id_, and select_mode_
    - Added constructors for single subnet and subnet range queries

    LeaseMgr
    Added virtual start query variants:
    - startSubnetLeaseStatsQuery4(SubnetID subnet_id);
    - startSubnetRangeLeaseStatsQuery4(SubnetID first, SubnetID last);
    - startSubnetLeaseStatsQuery6(SubnetID subnet_id);
    - startSubnetRangeLeaseStatsQuery6(SubnetID first, SubnetID last);

src/lib/dhcpsrv/tests/generic_lease_mgr_unittest.*
    GenericLeaseMgrTest
    - Added checkQueryAgainstRowSet()
    - Added testLeaseStatsQuery4()

src/lib/dhcpsrv/tests/lease_mgr_unittest.cc
    Added LeaseStatsQuery ctor tests
    - TEST (LeaseStatsQueryTest, defaultCtor)
    - TEST (LeaseStatsQueryTest, singleSubnetCtor)
    - TEST (LeaseStatsQueryTest, subnetRangeCtor)

src/lib/dhcpsrv/tests/memfile_lease_mgr_unittest.cc
    TEST_F(MemfileLeaseMgrTest, leaseStatsQuery4) {