[keyserver] rename retrieveAccountKeysSet -> retrieveSessionInitializationKeysSet

Summary:
Addresses: https://phab.comm.dev/D9422#276975

We get to the point where there are two functions with the same name, but doing different things. This should make it more consistent to avoid confusion.

Depends on D9427

Test Plan: Flow

Reviewers: marcin

Reviewed By: marcin

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9450