[lib][web][native] Rename callServerEndpoint to callSingleKeyserverEndpoint

Summary:
This function is going to stick around indefinitely in our codebase. Even if we get rid of all callsites in actions, it will continue to be needed within the internals of callKeyserverEndpoint. As such, I figured renaming it to clarify its function (as related to callKeyserverEndpoint) makes sense.

Depends on D10687

Test Plan:
I tested this stack using the following procedure:

1. I tested primarily on native
   1. I compiled a dev build and deployed it to an iOS simulator
   2. I created a brand new account on my local keyserver using the iOS app
   3. I ran Redux dev tools: cd native && yarn redux-devtools
   4. I added a 30s sleep at the start of resolveKeyserverSessionInvalidation
   5. I made KeyserverConnectionsHandler return null so that the socket wouldn’t automatically recover the session prior to my testing
   6. I killed the app
   7. I deleted all of the test user’s cookie
   8. I then opened the app again and navigated to a chat and sent two messages
   9. By following the Redux monitor, I was able to see that the keyserver session invalidation recovery was successful, and both messages were eventually sent after the 30s sleep concluded
2. On web, we don’t support keyserver session invalidation. However, I tested to make sure that the web app still loaded after my changes

Reviewers: tomek, inka, atul

Reviewed By: tomek, atul

Differential Revision: https://phab.comm.dev/D10688