Implementation of leader follower model for global resources

# Leader follower model for global resources

## Context

Global resources in TAO mean one ORB and using the reactive strategy one Reactor. TAO uses the ACE Select Reactor for this purpose. One main problem with multithreading and only one Reactor is that only one thread can wait in handle_events at a time. Handle_events is called, when a thread expects input, meaning it is blocking to wait for it.

## Idea

One solution to this problem is to use a leader-follower model, which partitions the set of threads wanting to wait for input into one leader and followers. Every thread is eligible to become a leader. A thread wanting to wait while nobody else is waiting becomes the leader. If the leader gets its input, which is in this case its response, it will select a new leader out of the set of followers. The followers wait on a condition variable they own and register it with the ORB core. The ORB core is thereby responsible for making this access thread-save by providing a lock for the list of followers with some flags, like leader_available.

## Implementation

The above mentioned condition variables are owned by the connection handlers, because reponses are expected per connection, not necessarily, thinking about other resource models, per thread.

The involved classes are TAO_ORB_Core, TAO_Client_Connection_Handler and TAO_ORB. In the TAO_ORB_Core class the list of followers, a lock, a leader reference counter and a leader thread ID were added. The handle_input and send_request methods on the TAO_Client_Connection_Handler contain 95% of the code used for implementing the leader-follower model. Care has to be taken, that the connection handlers are suspended and resumed, properly. A special concern is that no deadlocking occurs.

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