Send KILL commands to backends

KILL commands are now sent to the backends in an asynchronous manner. As
the LocalClient class is used to connect to the servers, this will cause
an extra connection to be created on top of the original connections
created by the session.

If the user does not have the permissions to execute the KILL, the error
message is currently lost. This could be solved by adding a "result
handler" into the LocalClient class which is called with the result.

include/maxscale/session.h

@@ -414,15 +414,6 @@ bool session_take_stmt(MXS_SESSION *session, GWBUF **buffer, const struct server
  */
 void session_clear_stmt(MXS_SESSION *session);
 
-/**
- * Try to kill a specific session. This function only sends messages to
- * worker threads without waiting for the result.
- *
- * @param issuer The session where the command originates.
- * @param target_id Target session id.
- */
-void session_broadcast_kill_command(MXS_SESSION* issuer, uint64_t target_id);
-
 /**
  * @brief Convert a session to JSON
  *