Better logging in case we can't compute the history prefix
Determine share id via the Horde_Share subsystem.

Calling getParameters() might be a bit faster
but this approach fails when the share has
never been "saved" via horde -> There is no annotation.

This change will most likely break the unit test
since we now depend on Horde_Share. Needs checking.
Only compute history prefix if we have changes to process
Log if we have to do a full history sync (which is normal for initial syncs)