RPC: add JSON-RPC batch request support

[redsh4de]

What

Add JSON-RPC 2.0 batch request support

Depends on #692

Why

Compliance with the spec, more efficient if you know you need to request several items at once

Where

cuprated, rpc

How

• /json_rpc takes raw Bytes instead of axum's Json; a body starting with [ is a batch, everything else is the existing single-request path. The Content-Type check Json did implicitly is now manual. This was done to improve batch performance
• A handrolled json content type checker was added so we don't have to depend on mime
• Entries are held as RawValue and parsed individually, so one bad entry only fails itself. Notifications are left out of the response array. An empty batch is not accepted
• Entries run concurrently via FuturesUnordered, capped by a new RpcHandler::batch_concurrency(). Responses come back in completion order, matched by id
• Docs: architecture book + both RPC READMEs