[keyserver][lib] Update freshMessageStore to return currentAsOf for all keyservers

Summary:
issue: https://linear.app/comm/issue/ENG-5722/update-freshmessagestore-function
freshMessageStore should return currentAsOf for all keyservers

Test Plan:
Tested that after the setInitialReduxState action the currentAsOf field is correct - a "map" from keyserver id to number.
This tests this function, because freshMessageStore is called by the keyserver in getInitialReduxStateResponder which returns the initial state. The states from all keyservers are then merged in getInitialReduxState, and the returnd value is dispatched with setInitialReduxState action.
I also logged to the console the value returned from freshMessageStore in getInitialReduxStateResponder and checked that it is correct.
I also logged in and checked that the currentAsOf in messageStore is correct after logInActionTypes.success

Reviewers: michal, kamil, ginsu

Reviewed By: michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9879