[4294] PostgreSQL now supports IPv4 lease stats recount

[4294] PostgreSQL now supports IPv4 lease stats recount

src/lib/dhcpsrv/cfg_subnets4.cc
    CfgSubnets4::updateStatistics() - removed lease mgr instance check

src/lib/dhcpsrv/mysql_lease_mgr.cc
    Added "ORDER by subnet_id" to RECOUNT_LEASE4_STATS

src/lib/dhcpsrv/pgsql_lease_mgr.cc
    Added tagged statement RECOUNT_LEASE4_STATS
    PgSqlAddressStatsQuery4 - new class, PostgreSQL derivation of AddressStatsQuery4
    PgSqlLeaseMgr::startAddressStatsQuery4() - PostgreSQL impl of virtual method

src/lib/dhcpsrv/srv_config.cc -
    SrvConfig::updateStatistics() - Added LeaseMgr singleton check around
    calls subnet statistics updates

src/lib/dhcpsrv/tests/cfgmgr_unittest.cc
    CfgMgrTest:
       ~CfgMgrTest() - now destroys LeaseMgr singleton
       startBackend(int family = AF_INET) -  new method to create memfile lease mgr

        TEST_F(CfgMgrTest, commitStats4)
        TEST_F(CfgMgrTest, commitStats6)
    - added call to startBackend()

src/lib/dhcpsrv/tests/pgsql_lease_mgr_unittest.cc
    TEST_F(PgSqlLeaseMgrTest, recountAddressStats4) - new test