Previously, all responses were converted to serde_json::Value before
being serialized to string. Since jsonrpc does not inspect the result
object in any way, this step could be skipped.

Now, result objects are serialized to string much earlier and the Value
step is skipped.

This patch has large performance benefits for huge responses: In tests
with 4.5 GB responses, the jsonrpc serialization overhead after
returning from an rpc function was reduced by around 35%. Which means
several seconds of speed-up in response times.

To accomplish this while mostly retaining API compatibility, the traits
RpcMethod, RpcMethodSync, RpcMethodSimple are now generic in their
return type and are wrapped when added to an io handler.

There's now a distinction between the parsed representation of jsonrpc
responses (Output/Response) and result of rpc functions calls
(WrapOutput/WrapResponse) to allow the latter to carry the
pre-serialized strings.

Review notes:
- I'm not happy with the WrapOutput / WrapResponse names, glad for
  suggestions.
- Similarly, rpc_wrap must be renamed and moved.
- The http handler health request is now awkward, and must extract the
  result/error from the already-serialized response. Probably worth it.
- The largest API breakage I could see is in the middleware, where the
  futures now return WrapOutput/WrapResult instead of Output/Result.
- One could make WrapOutput just be String, but having a separate type
  is likely easier to read.

See paritytech#212

- WrapOutput -> SerializedOutput
- WrapResponse -> SerializedResponse
- RpcMethodWrapped -> RpcMethodWithSerializedOutput

Co-authored-by: Tomasz Drwięga <tomusdrw@users.noreply.github.com>

Co-authored-by: Tomasz Drwięga <tomusdrw@users.noreply.github.com>