[5588] Cassandra shared lease stats support implemented

[5588] Cassandra shared lease stats support implemented

    Note this impl is done with calculate-on-demand SQL
    statements rather than stat tables pluse triggers.

src/lib/dhcpsrv/cql_lease_mgr.*
    Added new SQL statements for lease stats queries

    CqlLeaseStatsQuery
        Constructors - added variants to support where clause params

        start() - modified to support query variants based
        on where clause params

   CqlSqlLeaseMgr
        Added start variants:
        - startSubnetLeaseStatsQuery4(const SubnetID& subnet_id)
        - startSubnetRangeLeaseStatsQuery4(const SubnetID& first_subnet_id,
                                             const SubnetID& last_subnet_id)

src/lib/dhcpsrv/tests/cql_lease_mgr_unittest.cc
    New unit tests:
    - TEST_F(CqlLeaseMgrTest, leaseStatsQuery4)
    - TEST_F(CqlLeaseMgrTest, leaseStatsQuery6)