TcpAcceptor should be Clone like TcpStream

[alexcrichton]

If you've got a task in an accept() loop, there's currently no easy way to shut it down. This should take a similar approach to TcpStream and friends by supporting a close_accept() method which will force all accept loops to return.

This will notably allow for accept() on the same socket concurrently, which needs to be accounted for. Unlike concurrent reads/writes this should be pretty easy to deal with.

	libuv	native unix	native windows
TcpAcceptor::clone	✓	✓	✓
TcpAcceptor::close_accept	✓	✓	✓
UnixAcceptor::clone	✓	✓	✓
UnixAcceptor::close_accept	✓	✓	✓

WIP: https://github.com/alexcrichton/rust/tree/issue-15595

[thehydroimpulse]

This is exactly what I was looking for.