SocketManager: use a timer wheel

The maximum number of timers we need to support recently increased by an order
of magnitude, and the datastructures SocketManager was using were not efficient
at this scale. This change replaces the flat array with a timer wheel.

This increased performance in my barrier microbenchmark by 61%.
add timer_wheel dependency to utests
Merge into master from pull request #113:
SocketManager: use a timer wheel (https://github.com/floodlight/indigo/pull/113)