[zx] Add new API for zx::time

This also change zx::clock to be a class instead of a namespace, because
nested namespaces are banned.

TEST=Everything still compiles, tests still pass.

Change-Id: I954558aaaed554fe088c11ff7bbc7d914eebdf6f

[zx] Prepare to use typed clock for zx::time

This CL introduces zx::base_time, zx::time_utc and zx::time_thread as
alias to zx::time.

It also introduces a new templated factory method to produce
zx::base_time.

This is in preparation for the time type to be different depending on
the clock used. After this CL, clients will be able to use both the
current API, as well as the typed API. When all clients have migrated to
the new API, the old API will be removed and the time types will be
enforced.

TEST=Everything still compiles

ZX-2338

Change-Id: I3b6f4eced5df23b8dc4b1310bd8d0a89291b07a8

[tests][kstress] have test classes register dynamically at app start

Change-Id: I556d14c4a6b972f2c7811330e6371c5870557f00

[tests][kstress] add a timeout option and ctrl-c detection

Change-Id: I09d018aea46c1c4e173d7be88f6f4aa396f9537b

[kstress][vmstress] add some page faults to the mix, add verbose command line flag

Change-Id: Iaf222d71580c00f49c46f6a82d0dd70a48045a72

[tests][kstress] refactor the kernel stress test app to allow for additional test cases

Break the boilerplate test code into a class and have the main logic run
it.

Change-Id: I4be82dab3371ba53043d0ecb63e00d8c4c717e96