rollup merge of #23949: aturon/stab-timeout

rollup merge of #23949: aturon/stab-timeout

This commit renames and stabilizes:

* `Condvar::wait_timeout_ms` (renamed from `wait_timeout`)
* `thread::park_timeout_ms` (renamed from `park_timeout`)
* `thread::sleep_ms` (renamed from `sleep`)

In each case, the timeout is taken as a `u32` number of milliseconds,
rather than a `Duration`.

These functions are likely to be deprecated once a stable form of
`Duration` is available, but there is little cost to having these named
variants around, and it's crucial functionality for 1.0.

[breaking-change]

r? @alexcrichton

cc @sfackler @carllerche