implemented removal of servers

modified mysql_connpool_detach_connection to force closing a connection
modified structure of table "servers"
added function force_remove_servers() that sends mysql threads the command to force closing connections
extended enum mysql_server_status
added sess->active_transaction , but unsued yet
introduced 2 new global variables :
  mysql_poll_timeout_maintenance: poll() during maintenance
  mysql_maintenance_timeout : max duration of graceful maintenance
sqlite3_flush_servers_db_to_mem() now updates existing mysql_server
DUMP RUNTIME HOSTGROUPS now creates also table "runtime_servers"