Batching of requests from `KeyClaimManager.ensureSessionsForUsers()`

[hughns]

Currently the rust crypto will do a single key claim request irrespective of the number of users and devices requested:

matrix-js-sdk/src/rust-crypto/KeyClaimManager.ts

Lines 79 to 83 in a6fb753

	const claimRequest = await this.olmMachine.getMissingSessions(userList.map((u) => u.clone()));
	if (claimRequest) {
	logger.info("Making /keys/claim request");
	await this.outgoingRequestProcessor.makeOutgoingRequest(claimRequest);
	}

It would make sense to batch this at an appropriate batch size.

n.b. I don't know whether this should be done by making getMissingSessions() return multiple requests, or splitting the requests in KeyClaimManager. Presumably this behaviour might be of concern for the matrix-rust-sdk too.