A conceptional issue in `get_data_since` leads to incomplete
autoupdates. The behaviour was long time in the code, but only with a
lot of autoupdates (high concurrency) and the autoupdate delay I noticed
the bug during testing. I'm sure, that this issue might have caused
incomplete autoupdates (which the user may experience as "lost
autoupdates") in previous productive instances. Instead of quering a
range (from_change_id to to_change_id) one now can only get data from a
change id up to the max change id in the element cache. The max change
id gets now returned by `get_data_since`.
I also added a get_all_data with the capability of returning the
max_change_id at this point of time.
As a usability-"fix" (more like a fix the result of a bug, not the bug
itself) a refresh button for a poll was added, that issues an autoupdate
for the poll and all options.
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