Change SystemClockTime and SteadyClockTime duration type

All API methods should return time in nanoseconds to reduce unnecessary
casts outside the Api class.
Add mutableSystemClockNow and mutableSteadyClockNow methods
Remove setSystemClockNow and setSteadyClockNow methods

in favor of the mutableSystemClockNow and mutableSteadyClockNow methods.
Add contains and find helper functions for containers
Add Trigger type alias and component types
Add Proactor abstract class
Add FileDescriptorSetFake test helper class
Add PselectApiStub test helper class
Add Awaiter abstract class with timeout implementation
Merge pull request #91 from magicant/feature/proactor

Partial implementation of awaiter (timeout)