rpc: Implement websockets with github.com/gorilla/websocket

[trinhdn2]

This change makes package rpc use the github.com/gorilla/websocket package
for WebSockets instead of golang.org/x/net/websocket. The new library is more robust
and supports all WebSocket features including continuation frames.

There are new tests for two issues with the previously-used library:

• TestWebsocketClientPing checks handling of Ping frames.
• TestWebsocketLargeCall checks whether the request size limit is applied correctly.

DialWebsocket's handling of the default Origin header has changed. It used to set the
local machine's hostname as the origin, but that's weird and won't work with any
Internet-facing service. The new behavior is creating connections without an Origin
header unless one is provided explicitly.

This change also raises the request size limit for HTTP and WebSocket connections
to 5 MB. Quite a few people have reported issues where geth fails to deploy contracts
when they're too large, and retesteth needs a larger limit to upload the pre-state of large
tests.

References:

• rpc: implement websockets with github.com/gorilla/websocket  ethereum/go-ethereum#19866