[lib][native] Make cancellation work for keyserver session recovery

Summary:
Cancellation for an authoritative keyserver session recovery will result in SET_NEW_SESSION getting called, since resolveKeyserverSessionInvalidation will return null.

In later diffs, we'll introduce a "connection issue" for non-authoritative keyserver session recovery. In that case, cancellation should just result in a new recovery being attempted.

Depends on D11222

Test Plan:
In addition to the plan described here: https://gist.github.com/Ashoat/75ab690d5c53cdd68a51b02e03e27c58

I also tested by adding a cancel() call and confirming that it made session recovery fail

Reviewers: inka, tomek

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D11223