[SDK] Key querying is not chunked and servers might not support an arbitrary amount of users in the request.

[manuroe]

From element-android created by poljar: element-hq/element-android#2925

Describe the bug
During my testing using Complement with this PR matrix-org/complement#75, I hit a bug when element would need to send out a /keys/query request. Complement creates an server with N E2EE capable users that are joined into a single room.

When I increased the number of users to 1000, synapse failed to return a response for the initial /keys/query request which contains all 1000 users.

To Reproduce
Steps to reproduce the behavior:

1. Freshly log in to an account that shares an E2EE room with 1000 users.
2. Try to send a message in the room.
3. Observe the 500 /keys/query response in the network tab.

This completely wedges the E2EE support and the message can't ever be sent.

I'm not sure if this is a specific limitation of the Synapse install Complement was running but it seems a good idea to chunk /keys/query requests anyways. It seems that chunking the requests out at 200 or 500 users (or anything between that) would work.

Note that this is the same problem as matrix-org/matrix-js-sdk#1619