[WIP (tests/locking?)] Update read and starred messages methods

[neiljp]

This is intended to add read-flag updates sent from the server, so if messages are read in the webapp and ZT is running, the intention is that the unread counts and status in ZT should update at the same time. It also extends the capability of the similar starred feature to cover where multiple message updates occur at once, in line with the read-flag version.

I would appreciate any testing of this PR :)

CAVEATS:

• On reading many messages quickly (in the webapp), the numbers can become out of sync with ZT (is this due to some kind of locking issue? multiple messages sent from the server which aren't responded to?)
• Some stream totals are occasionally wrong on loading?
• I'd like to add some tests for the new method, but wanted to look for feedback first.