Currently http cors and websocket origins are a comma separated string in the
config object. These are replaced with string arrays that are more expressive in
case of a config file.

There is no need to depend on the old context package now that the
minimum Go version is 1.7. The move to "context" eliminates our weird
vendoring setup. Some vendored code still uses golang.org/x/net/context
and it is now vendored in the normal way.

This change triggered new vet checks around context.WithTimeout which
didn't fire with golang.org/x/net/context.

Fixes #3762.  Details about parameter:
https://github.com/rs/cors/blob/a62a804a8a009876ca59105f7899938a1349f4b3/cors.go#L50-L54

The new client implementation supports concurrent requests,
subscriptions and replaces the various ad hoc RPC clients
throughout go-ethereum.

rpc: be less restrictive on the request id
rpc: improved documentation
console: upgrade web3.js to version 0.16.0
rpc: cache http connections
rpc: rename wsDomains parameter to wsOrigins

Separated by spaces

When Id is missing, the client does not want a response

New RpcConfig object to pass growing config

http://www.jsonrpc.org/specification#batch

- camelcase fields dont play nice with otto value magic: JsonRpc -> Jsonrpc, ID -> Id
- jeth: ethereum.js rpc transport provider direct bridge between js and ethereumApi via otto jsre

Allow returning different JSON RPC error codes depending on error type