changes arising from Nordic work

Here’s a compilation of some of the changes I want to make to mbed-src. They have mostly to do with power optimizations originating from the Nordic work. Here are the highlights:

-   Replace the use of high-frequency clock with low-frequency crystal oscillator in system startup.
-   Rebase the implementation of us-timer (micro-second timer) on top of the app_timer module provided by Nordic (which uses the RTC).
-   Introduce timestamp_t as a type to represent time within TimerEvent. This helps avoid complications arising from timer wrap-around.
-   Minor improvements to the ticker implementation.

Thanks,