Chrono cleanup

[K-ballo]

Our code base currently supports both boost::posix_time and boost::chrono, forwarding the later to the former. We should remove support for boost::posix_time and build on top of boost::chrono (and some day std::chrono).

Some thoughts on the subject:

• Examples and unit tests should be updated to use the chrono interfaces.
• All functionality should be implemented on top of chrono::steady_clock, since operations should not be affected by system clock changes.
• Instead of having multiple implementations, there should be a single implementation in terms of chrono::steady_clock::time_point. The interface should support arbitrary durations and time-points (including our take on executors).
• Timed operations shall not do any non-trivial operation (holding a lock, allocating memory, etc) when the given timeout has already expired.
• Add support for std::chrono, forwarding/converting to boost::chrono.

[K-ballo]

Turns up there's not much to do on timed operations with expired timeouts, as we need to suspend/yield anyways.