rpc-v2: Limit transactionBroadcast calls to 16

[lexnv]

This PR limits the number of active calls to the transactionBroadcast APIs to 16.

cc @paritytech/subxt-team

Closes: #3081

[jsdw]

I wonder why this type annotation needed adding!

[lexnv]

This is an edge-case in the rust compiler, where the rustc cannot deduce the type of this stream.

I'm not entirely sure what is happening behind the scenes, I expect the async block cannot be properly deduced here.

The error I get is:

error[E0308]: mismatched types
   --> substrate/client/rpc-spec-v2/src/transaction/transaction_broadcast.rs:222:3
    |
146 |                 |notification| async move { notification.is_new_best.then_some(notification.hash) },
    |                                --------------------------------------------------------------------
    |                                |
    |                                the expected `async` block
    |                                the found `async` block
...
222 |         sc_rpc::utils::spawn_subscription_task(&self.executor, fut);
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
    |
    = note: expected `async` block `{async block@substrate/client/rpc-spec-v2/src/transaction/transaction_broadcast.rs:146:20: 146:88}`
               found `async` block `{async block@substrate/client/rpc-spec-v2/src/transaction/transaction_broadcast.rs:146:20: 146:88}`
note: the lifetime requirement is introduced here
   --> /home/lexnv/workspace/polkadot-sdk/substrate/client/rpc/src/utils.rs:178:34
    |
178 |     fut: impl Future<Output = ()> + Send + 'static,
    |                                     ^^^^

For more information about this error, try `rustc --explain E0308`.

[jsdw]

A couple of small nits but all makes sense to me!

I don't mind the limit being 4 just to exercise clients and nudge them to be able to handle a max of 4, but for most other cases I wonder what an actually sensible limit would be here

[github-actions]

Review required! Latest push from author must always be reviewed

[niklasad1]

we could probably change the return type Option<String> because this error is never used by the impl

however, https://github.com/paritytech/jsonrpsee/blob/master/core/src/server/mod.rs#L153-#L171 IntoResponse is not implemented for Option<String> so never mind.