[ws-server] Batch support (#300)

* Batch requests over the websocket

* Use same call signature for both http and ws server

* Use CallError instead of InvalidParams
Add a draft batch request test

* Sort out formatting of batch responses
Cleanup
Use CallError

* Add test for slow method calls

* Update utils/src/server.rs

Co-authored-by: Niklas Adolfsson <[email protected]>

* Add benchmark for batched websocket requests

* rename

* Rename batch channels

* Sort out the InvalidParams situation

* Remove one level of InvalidParams
Return app-level error when call fails

* Update ws-server/src/server.rs

Co-authored-by: Andrew Plaza <[email protected]>

* Update ws-server/src/server.rs

Co-authored-by: Andrew Plaza <[email protected]>

Co-authored-by: Niklas Adolfsson <[email protected]>
Co-authored-by: Andrew Plaza <[email protected]>