Add EventLoop.now API for getting the current time

[simonjbeaumont]

Motivation

Code that tries to work with NIODeadline can be challenging to test using EmbeddedEventLoop and NIOAsyncTestingEventLoop because they have their own, fake clock.

These testing event loops do implement scheduleTask(in:_) to submit work in the future, relative to the event loop clock, but there is no way to get the event loop's current notion of "now", as a NIODeadline, and users sometimes find that NIODeadline.now, which returns the time of the real clock, to be surprising.

Modifications

Add EventLoop.now to get the current time of the event loop clock.

Result

New APIs to support writing code that's easier to test.

[glbrntt]

One typo, but looks good otherwise!