When a thread has been idle for 10 minutes and we have too many threads
in the threadpool the thread will be pruned. This is done by the
thread's main function just returning. Unfortunately this does not free
up the memory, the thread either needs to be joined or detatched.
We cannot use detached threads since there are mutexes and conditional
variables between the main thread and the threadpool threads. If the
main thread finishes before the threadpool threads (as would happen in
cpimport) then crashes occur. The parent needs to wait on the child
threads which is the whole point in joining.
So this fix spawns a new thread which every minute will check the list
of threads to be joined due to timeout and join them.
We have had to use an adapted version of boost::thread_group so that we
can join a single thread based off its thread ID.
In addition with have modified PriorityThreadPool to use detached
threads since this does not need to signal the child threads at the end.
PriorityThreadPool didn't have very good error handling. If something
failed it would just ignore whatever was being processed. This could
lead to a query continuing without retreiving all of the required data.
This patch adds error handling, sending a message back to the client
and a log message. It also destroys and recreates the pool thread.
