Allow concurrent readers

[nhooyr]

It'd be nice if concurrent readers worked the same way concurrent writers do.

i.e right now if I wanted to spawn a pool of goroutines to read messages from a WebSocket and respond to them, I'd need to spawn an extra goroutine to read the messages and then pipe them to the workers via a channel. It'd be simpler if the library just automatically blocked additional Reader calls until previous readers were closed.

[nhooyr]

Also need to ensure NetConn can be accessed concurrently on all methods, it doesn't look like it Read at least can.

[nhooyr]

Looks like the reason I never allowed this is because you want to set a timeout between every message, not a timeout on a single worker waiting for a message. Fixed the NetConn bug though.