ThreadedIoFile spawns a new thread for every new file. Thread
information is stored for tracking purpose by base::tracked_objects.
The tracking object remains even if the thread itself is destroyed.
This results in memory usage increased by a couple of bytes for every
new segment created in live mode (new segments spawns new threads).
Use WorkerPool instead to avoid spawning new threads.
Fixes Issue #61.
Change-Id: Id93283903c3ba8ebf172a0d58e19b082a72c6cf0
b17d240060