[ws server] async method calls

[FelipeRosa]

Closes #422

Hey folks,

I thought of this fix for the problem I reported in #422.

I don't know whether you have preferences of where that tokio::spawn should occur. Since JsonRpcRequest is borrowed, I decided making the tokio::spawn call before parsing the incoming request so data gets moved inside the async block.

Feel free to make any edits you think are appropriate.

[niklasad1]

thanks looks good, but I want to hear if @maciejhirsz has any objections on the overhead of tokio::spawn in this context or if we should tokio::spawn inside execute/rpc module instead?!

[maciejhirsz]

I think between this and #408 it would make most sense to just always spawn async calls on tasks. We already need have RpcParams for that so there is no need to allocate a separate buffer for each message (plus using a shared buffer saves us from reallocation issues).

[maciejhirsz]

@FelipeRosa thanks for the PR and bringing our attention to this! I think now that #424 is merged it should have solved your use case so I'll close this PR. Feel free to re-open it or file an issue if there is anything we might have missed.