[5477] dhcpsrv/postgres and kea-dhcp4 now support db reconnect

[5477] dhcpsrv/postgres and kea-dhcp4 now support db reconnect

kea-dhcp4
    added support for max-reconnect-tries and reconnect-wait-time
    to lease and host db parsers

    Added a callback for when DB backends detect loss of connectivity

    Added a self-rescheduling method to attempt to reconnect to the
    backends if retries are enabled

dhcpsrv
    Added a callback that DatabaseConnection derivations should invoke
    when they lose connectivity.

    Added an optional callback parameter from CfgDbAccess::createManagers()
    all the way down to DatabaseConnection ctor.

    pgsql_connection.cc
        PgSqlConnection::~PgSqlConnection() - Added logic to close the
        connection only when the connect state is still OK.
        Otherwise it likes to core dump.

        PgSqlConnection::checkStatementError() - Modified to invoke the
        connectivity lost callback on "fatal" errors

    pgsql_lease_mgr_unittest.cc
    pgsql_host_data_source_unittest.cc
        Added tests to verify that the lost callback is NOT invoked on an
        open failure