[lib] Fix currentAsOf

Summary: If the keyserver was not present in the message store previously, processedMessageStore.currentAsOf[keyserverID] will be undefined. And max from number and undefined is NaN

Test Plan: Tested that now when a user logs in to a new keyserver and some messages are sent from it, the value is not NaN

Reviewers: michal, kamil, ginsu, ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10652