[5556a] MySQL lease and host backends now support configurable auto-reconnect

[5556a] MySQL lease and host backends now support configurable auto-reconnect

    src/lib/dhcpsrv/mysql_connection.h
        MySqlConnection::checkError<>() - modified to invoke
        db lost callback

    src/lib/dhcpsrv/dhcpsrv_messages.mes
        Updated log messages

    src/lib/dhcpsrv/mysql_lease_mgr.cc
        MySqlLeaseMgr::getVersion() - updated to use checkError()

    src/lib/dhcpsrv/pgsql_connection.*
        PgSqlResult::PgSqlResult(PGresult *result) - now supports
        construction with null PGresult. This is to accomodate rare
        cases when PQ* statements can return NULL.

    src/lib/dhcpsrv/tests/generic_lease_mgr_unittest.*
        class LeaseMgrDbLostCallbackTest - new test fixture for
        testing LeaseMgr DBLostCallback behavior

    src/lib/dhcpsrv/tests/host_mgr_unittest.cc
        class HostMgrDbLostCallbackTest
        class MySQLHostMgrDbLostCallbackTest
        class PostgreSQLHostMgrDbLostCallbackTest
        - new test fixtures for testing HostMgr DBLostCallback behavior

    src/lib/dhcpsrv/tests/mysql_lease_mgr_unittest.cc
        class MySQLLeaseMgrDbLostCallbackTest - new test fixture for
        testing MySQL LeaseMgr DBLostCallback behavior

    src/lib/dhcpsrv/tests/pgsql_lease_mgr_unittest.cc
        class PgSqlLeaseMgrDbLostCallbackTest - new test fixture for
        testing Postgresql LeaseMgr DBLostCallback behavior

    src/lib/dhcpsrv/tests/test_utils.*
        int findLastSocketFd() - new function used for finding what
        should be the fd of the SQL client socket

    doc/guide/dhcp4-srv.xml
    doc/guide/dhcp6-srv.xml
        Updated lease and host database parameter sections