fixed a bug with incremental taxonomy update: when concepts become
equivalent after addition, taxonomy was updated incorrectly
differential index: deletions are now committed immediately to the main
index, but also kept separately for initialization of deletions
differential index: deletions to object cached are kept separately and
committed only after deletions are applied (this is needed to be able to
iterate over all contexts for deletions initialization)
the logger for tests will print warning and error messages to sterr
currently not all contexts are cleaned by the cleaning stage (but all
tests, nevertheless work): this needs to be further investigated; debug
code is currently in place
Merge branch 'elk-new-incremental' of https://github.com/klinovp/elk.git into elk-new-incremental