Move context to first parameter in DialContext

Follows best practice and pkg/context documentation:

> The Context should be the first parameter, typically named ctx

Improve server subprotocol documentation

Partial fix for 404.

Reduce memory used in chat example

pool flate writers

Handle invalid close frames

Send protocol error if close code or payload are invalid.

Fixes Autobahn tests 7.5.1, 7.9.*.