Aggregations: Fixes pre and post offset serialisation for histogram aggs

Changes the serialisation of pre and post offset to use Long instead of VLong so that negative values are supported.  This actually only showed up in the case where minDocCount=0 as the rounding is only serialised in this case.

Closes #7312
The problem here is that it would break multi-version clusters. We still need to read/write vLong depending on in/out.getVersion so that at least positive offsets work.