Don't ask mgm server for port to connect transporter to if we already have one; only ask if connect fails.

Reconnect to mgm server in connect_clients thread if m_mgm_handle is disconnected.


ndb/include/mgmapi/mgmapi.h:
  Add prototype for ndb_mgm_is_connected
ndb/src/common/transporter/TransporterRegistry.cpp:
  start_clients_thread:
  - don't get port if we don't have to
  - Reconnect to mgm server if m_mgm_handle is disconnected
ndb/src/mgmapi/mgmapi.cpp:
  add ndb_mgm_is_connected