ThreadPool should use std::deque instead of std::list to hold _pendingTasks

While profiling network_interface_asio_integration_test, which uses a ThreadPool, I found that ~60% of cycles were spent in ThreadPool::schedule(). Changing _pendingTasks from a list to a deque made schedule() take a more reasonable amount of time. The culprit is probably libstdc++ not implementing list::size as an O(1) operation, despite that being mandated by the C++11 standard.