Reduce verbosity of chunk merge change log event

After committing a chunks merge operation we emit a change log event containing all the merged chunks. Thus if we are merging 1000 chunks the relative event will contain a list of 1000 chunks.
I think there is no need to list all the merged chunks and instead we could simply include in the log event:

• The new merged chunks from which we can infer the range (min/max) that we merged
• Total number of merged chunks.