WS pubsub example.

[tomusdrw]

• Exposing futures::sync::mpsc::Sender in RequestContext
• Apparently ws::Sender doesn't track if the connection is still active so I've added manual tracking and exposed it in RequestContext.
• Updates all examples to avoid glob imports (only jsonrpc_core is imported with a wildcard)

Closes: #120

[faern]

@tomusdrw Not using wildcard imports (use jsonrpc_core::*;) would make it easier for newcomers to track where types used in the example come from.

[tomusdrw]

@faern Thanks for the suggestion. Updated all examples.

[faern]

@tomusdrw Is the add_method/add_subscription or the macros approach the preferred usage of this crate? Are both meant to be supported for a long time? I got the impression that the macros support is the new kid on the block and was a bit surprised that the new example then uses the other way.

[tomusdrw]

pubsub crate is more low-level (works on raw Param, Value and method level), since it's a custom extension to JSON-RPC protocol we want to keep it separate.

macros crate is designed to create a complete services on top of those low-level crates, but uses a bit of macro magic currently.

That said both methods are equally supported, our main driver for this library is development of RPC interfaces in https://github.com/paritytech/parity and we actually use macros there.

The example uses add_method stuff cause it's in pubsub crate, feel free to add another example in macros crate to show WS + PubSub + Macros.

[faern]

Using macros instead was pretty easy. I don't think there is any information there that is not already available in the existing PubSub + Macros examples. Getting this merged as is with the added Sender/RequestContext functionality will satisfy my needs for now.