Update strategy for retrying failed event indexing (broken)

This is an alternate strategy to the too simplistic strategy that was in
place before. The idea with this strategy is that every time a new event
ID is retrieved from the DB for the indexer, we write that ID as well as
the previous ID into a file on disk. At the end of a successful batch of
event indexing, we clear the previous ID property.

This way, when starting a new batch of events for indexing, we can know
that, if the previous ID property is set, then the last batch of
indexing did *not* complete successfully. Thus, we retry starting from
the previous ID instead of the current ID.

The reason that this is "broken" is that it seems that sometimes the
indexing process experiences an OOM exception, but it gets caught
somewhere in the executor stack. As a result, the FullTextIndexer
continues working on the next event ID without recognizing that the
previous event ID needs to be retried.