[5586] MySQL shared leases stats implemented

[5586] MySQL shared leases stats implemented

    src/share/database/scripts/mysql/dhcpdb_create.mysql
    src/share/database/scripts/mysql/dhcpdb_drop.mysql
    src/share/database/scripts/mysql/upgrade_5.2_to_6.0.sh.in
        Added MySQL v4/6 stat tables and triggers

    src/lib/dhcpsrv/memfile_lease_mgr.cc
        Suppress output of of rows with count values of 0

    src/lib/dhcpsrv/mysql_lease_mgr.*
        Added v4/v6 lease stat SQL statements

        MySqlLeaseStatsQuery
        - Added ctor variants that accomodate selection criteria
        - Modified start() to handle three variants of selection

        MySqlLeaseMgr
        - Added start***Query4/6 variants

    src/lib/dhcpsrv/tests/mysql_lease_mgr_unittest.cc
        Added tests:
        - TEST_F(MySqlLeaseMgrTest, leaseStatsQuery4)
        - TEST_F(MySqlLeaseMgrTest, leaseStatsQuery6)