Use fixed max senders/receivers due to 10MB limit for max_response_body_size

[Dengjianping]

Signed-off-by: Dengjianping djptux@gmail.com

Currently, there's a limit on the max_response_body_size, which is 10MB(10485760 bytes).
So by calculation, 10MB means

let (max_senders, max_receivers) = (1024 * 16, 1024 * 16);

Before serialization, the data in memory is about 3.39MB.
But after serialization, it will become 9.88MB, almost 10MB.

So if any number exceeds 1024 * 16, the server will return an error like this

The background task been terminated because: Networking or low-level protocol error: WebSocket connection error: 
message too large: len >= 12951047, maximum = 10485760; restart required

Take a look at this PR: paritytech/jsonrpsee#711
and https://github.com/paritytech/jsonrpsee/blob/v0.15.1/core/src/lib.rs#L81

So we have to use fixed values for (max_senders, max_receivers).

jsonrpsee is using serde for serialization/deserialization, seems there's no way to change to another crate like codec for more compacted data.

[flame4]

LGTM!