native: Implement clone/close_accept for unix

This commits implements {Tcp,Unix}Acceptor::{clone,close_accept} methods for
unix. A windows implementation is coming in a later commit.

The clone implementation is based on atomic reference counting (as with all
other clones), and the close_accept implementation is based on selecting on a
self-pipe which signals that a close has been seen.
rustuv: Implement clone/close_accept

This commits implements {Tcp,Unix}Acceptor::{clone,close_accept} methods for
all of librustuv.

This implementation rewrites much of Access, AccessTimeout, and AcceptTimeout to
have type parameter for shared state that all acceptors share (a shared queue of
sockets). The incoming/outgoing channels have been removed as all timeouts and
such are now managed on the event loop rather than concurrently.
native: TCP close/close_accept for windows

This commit implements TcpAcceptor::{close, close_accept} for windows via
WSAEVENT types.
native: Add some documentation for accept

Document the new code for how close_accept and timeouts are implemented.
native: clone/close_accept for win32 pipes

This commits takes a similar strategy to the previous commit to implement
close_accept and clone for the native win32 pipes implementation.

Closes #15595
auto merge of #15704 : alexcrichton/rust/issue-15595, r=brson

If a task is spinning in an accept loop, there is currently no method of gracefully shutting it down. This PR introduces a way to do so by cloning the acceptor and implementing a close_accept method to unblocking any pending acceptor.

As with other I/O methods like this, it is `#[experimental]` from the start and sadly carries with it a good deal of code to support it. Much of the complication is from the fact that you can now concurrently accept on the same socket.

I tried to add a good deal of tests for this change, but another set of eyes is always appreciated!