Make `try_recv` do what you expect

[brson]

The obvious semantics for try_recv is to return a message or return immediately if none is available (i.e. peek + recv). That's not what it does though; instead it blocks waiting for a message and returns None when the sender closes the channel.

cc #10138

Nominating.

[brson]

Could possibly get rid of peek then if nobody is using it for anything other than peek + recv.

[pnkfelix]

Delegating to @alexcrichton as well.

[alexcrichton]

Closing in favor of #10459