Read-only transaction processing - prevent thread starvation while in the read window.

[greg7mdp]

This is an issue related to the processing of read-only transactions in parallel during the read window.

When starting the read window, a task is posted for each thread in the ro thread pool (_ro_thread_pool), and this task has a loop doing approximately:

while (/* time left in read window */) {
      if (ro_trx_queue.empty())
          break;
      // pop and execute ro transaction
      ...
}

The issue is that, if for example we have only one ro trx in the queue when we enter the read window, the first thread will start executing this transaction, and all the other tasks will exit, leaving a single task to execute all further read transactions which may be posted while the read window is still active.

So essentially, for the remainder of the read window we would effectively execute readonly transactions on a single thread.

Instead of exiting the task, if there are other threads currently executing ro transactions, and there is time left in the read window, we should probably wait on a condition variable associated to the queue (_ro_trx_queue) instead of exiting the loop.

This will require changes to _ro_trx_queue. In addition encapsulate queue lock/unlock by adding a push/pop members to _ro_trx_queue.