[4294] Memfile and MySql now support recalulating IPv4 lease statistics

[4294] Memfile and MySql now support recalulating IPv4 lease statistics

src/lib/dhcpsrv/cfg_subnets4.cc
    CfgSubnets4::removeStatistics()
    - added removal of all lease statistics per subnet, and global declined address
    stats

    CfgSubnets4::updateStatistics()
    - added call to LeaseMgr::recountAddressStats4

src/lib/dhcpsrv/lease.cc
src/lib/dhcpsrv/lease.h
    Replaces lease state constants with LeaseState enumeration.

src/lib/dhcpsrv/lease_mgr.cc
src/lib/dhcpsrv/lease_mgr.h
    struct AddressStatsRow4 - contains the content of one row of the IPv4
    lease statistical data result set

    class AddressStatsQuery4 - base class for constructing the IPv4
    lease statistical data result set for an IPv4 lease storage

    LeaseMgr::recountAddressStats4() -  new method which recalculates
    per-subnet and global stats for IPv4 leases

    LeaseMgr::startAddressStatsQuery4() - new virtual method that fetches
    the IPv4 lease statistical data result set

src/lib/dhcpsrv/lease_mgr_factory.h
src/lib/dhcpsrv/lease_mgr_factory.cc
    LeaseMgrFactory::haveInstance() - new static method which indicates
    whether or not the lease manager singleton exists

src/lib/dhcpsrv/memfile_lease_mgr.h
src/lib/dhcpsrv/memfile_lease_mgr.cc
    MemfileAddressStatsQuery4 - Derivation of AddressStatsQuery4, it constructs
    the IPv4 lease statistical data by iterating over IPv4 lease storage

    Memfile_LeaseMgr::startAddressStatsQuery4() - new virtual method which
    creates, starts, and returns a MemfileAddressStatsQuery4

src/lib/dhcpsrv/memfile_lease_storage.h
    Added an a per subnet_ID index to IPv4 storage

src/lib/dhcpsrv/mysql_lease_mgr.h
src/lib/dhcpsrv/mysql_lease_mgr.cc
    - Added RECOUNT_LEASE4_STATS query

    MySqlAddressStatsQuery4 Derivation of AddressStatsQuery4, it constructs
    the IPv4 lease statistical data by executing RECOUNT_LEASE4_STATS

    MySqlLeaseMgr::startAddressStatsQuery4() - new virtual method which
    creates, starts, and returns a MySqlAddressStatsQuery4

src/lib/dhcpsrv/tests/alloc_engine_utils.cc
    AllocEngine6Test::AllocEngine6Test()
    AllocEngine4Test::AllocEngine4Test()
    - moved lease mgr create up above configuration commit

src/lib/dhcpsrv/tests/cfg_db_access_unittest.cc
     ~CfgMySQLDbAccessTest() - added destruction of lease manager singleton,
    otherwise subsequent tests can fail

src/lib/dhcpsrv/tests/generic_lease_mgr_unittest.h
src/lib/dhcpsrv/tests/generic_lease_mgr_unittest.cc
    GenericLeaseMgrTest::checkStat() - new method for comparing a stat
    GenericLeaseMgrTest::checkAddressStats4() - new method for comparing a list
    of stats
    GenericLeaseMgrTest::makeLease4() - new method for making a minimal lease
    GenericLeaseMgrTest::testRecountAddressStats4() - new method which tests
    a lease manager's ability to recalculate the IPv4 lease statistics

src/lib/dhcpsrv/tests/memfile_lease_mgr_unittest.cc
    TEST_F(MemfileLeaseMgrTest, recountAddressStats4) - new test which tests
    Memfile_LeaseMgr's ability to recalculate IPv4 lease statistics

src/lib/dhcpsrv/tests/mysql_lease_mgr_unittest.cc
    TEST_F(MySqlLeaseMgrTest, recountAddressStats4) - new test which tests
    MySqlLeaseMgr's ability to recalculate IPv4 lease statistics