Protection against very large queries

There are cases where system runs out of memory because very large queries (5 MB / around 30'000 lines of text each) were run and QueryStatsMbean consuming huge memory.

We should have some protection against such queries. Options are:

• We can log a warning if the query is longer than e.g. 100 KB.
• A query longer than e.g. 1 MB might just throw an exception.
• The query statistic should probably not collect statistic for such long queries. We could shrink the query text in the statistic collection to e.g. at most 2 KB per query. Truncating it, or, probably better, only retaining the first and last 1 KB.