[keyserver][native][web] Use FCCache instead of calling fetchFarcasterChannelByID directly

Summary:
Noticed some places we could be using the cache but aren't.

Depends on D13974

Test Plan: Tested in combination with later diffs

Reviewers: varun, will

Reviewed By: varun

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13975