spawned threads with a reusable class Thread.

This is the second idea implemented in the Alik's patch for
BUG#22306: STOP INSTANCE can not be applied for instances in Crashed,
Failed and Abandoned.
Commiting separately to ease review process. 


server-tools/instance-manager/commands.cc:
  Remove an unused header.
server-tools/instance-manager/guardian.cc:
  Use Thread framework instead of manually spawning the Guardian thread.
  Tidy up.
server-tools/instance-manager/guardian.h:
  Use Thread framework instead of manually spawning the Guardian thread.
server-tools/instance-manager/instance.cc:
  Use Thread framework instead of manually spawning the instance
  monitoring thread.
server-tools/instance-manager/listener.cc:
  Use Thread framework instead of manually spawning the 
  mysql connection thread.
server-tools/instance-manager/listener.h:
  Use Thread framework instead of manually spawning the 
  mysql connection thread.
  Rename Listener_thread to Listener for brevity.
server-tools/instance-manager/manager.cc:
  Change references to pointers, as per the coding style.
  Use Thread framework instead of manually spawning threads.
server-tools/instance-manager/mysql_connection.cc:
  Get rid of Mysql_connection_thread_args. Use class Thread framework
  instead. Rename Mysql_connection_thread to Mysql_connection for brevity.
server-tools/instance-manager/mysql_connection.h:
  Get rid of Mysql_connection_thread_args. Use class Thread framework
  instead. Rename Mysql_connection_thread to Mysql_connection for brevity.
server-tools/instance-manager/priv.cc:
  Move set_stacksize_and_create_thread to thread_registry.cc and make it
  static: it is not used anywhere else now.
server-tools/instance-manager/priv.h:
  No public set_stacksize_n_create_thread
server-tools/instance-manager/thread_registry.cc:
  Implement a base Thread class to be used for all Instance Manager
  threads.
server-tools/instance-manager/thread_registry.h:
  Implement a base Thread class to be used for all Instance Manager
  threads.