thread pool: add minimum size of 4 threads

rationale: a minimum thread pool size of N allows up to N dependent
tasks to block on each other without resulting in a deadlock on monothread
systems, while limiting the number of concurrent running tasks.
Ring currently uses up to two dependent tasks with ThreadPool.

Change-Id: I66ec6ebb64ee4e1fb84af2db9aa465c62e08eadd

src/thread_pool.cpp

@@ -32,7 +32,7 @@ struct ThreadPool::ThreadState
 };
 
 ThreadPool::ThreadPool()
- : maxThreads_(std::thread::hardware_concurrency())
+ : maxThreads_(std::max<size_t>(std::thread::hardware_concurrency(), 4))
 {
     threads_.reserve(maxThreads_);
 }