[keyserver][lib] Introduce fetch_pending_updates endpoint

Summary:
This addresses ENG-8088. In a later diff, the client will call this when encountering a socket crash loop. The higher timeout will allow us to download a larger amount of data.

Depends on D12363

Test Plan:
In combination with later diffs, I tested as follows:

1. I created a socket crash loop on a physical iOS device using @inka's create-many-threads-to-trigger-crash-loop.js script (see ENG-8090
2. I confirmed that the socket was unable to connect prior to my diff stack
3. I confirmed that after applying my diff stack, the SUCCESS action was dispatched, and the socket was able to connect afterwards

Reviewers: tomek, kamil

Reviewed By: tomek, kamil

Subscribers: kamil, inka

Differential Revision: https://phab.comm.dev/D12364