Enables the timer scheduler's customization for both deadline and waitable timers

[securesocketfunneling]

This pull request is linked to pull request discussion #24

The discussion on timer precisions with @haegele-tv lead us to think that we have to design our own timer mechanism for the boost asio udt implementation requirements.

The perfect place to do so would be to design a specific timer scheduler (as it is done with WinRT, or Linux, Windows...) and benefit of the entire deadline/waitable timer io_objects and io_services architecture.

Yet, the timer scheduler is NOT customizable by the user. This patch allows third party developers to specify a custom timer scheduler for any specific requirements.

What do you think?

[securesocketfunneling]

Here is a branch using custom timer_scheduler:

Timer_benchmark tool on develop-high-precision-timer-scheduler branch.

This could be achieved using timer_scheduler customization as this PR suggests.