Reduce heap pressure during compactions

Small partition compactions are painfully slow with a lot of overhead per partition. There also tends to be an excess of objects created (ie 200-700mb/s) per compaction thread.

The EncoderStats walks through all the partitions and with mergeWith it will create a new one per partition as it walks the potentially millions of partitions. In a test scenario of about 600byte partitions and a couple 100mb of data this consumed ~16% of the heap pressure. Changing this to instead mutably track the min values and create one in a EncodingStats.Collector brought this down considerably (but not 100% since the UnfilteredRowIterator.stats() still creates 1 per partition).

The KeyCacheKey makes a full copy of the underlying byte array in ByteBufferUtil.getArray in its constructor. This is the dominating heap pressure as there are more sstables. By changing this to just keeping the original it completely eliminates the current dominator of the compactions and also improves read performance.

Minor tweak included for this as well for operators when compactions are behind on low read clusters is to make the preemptive opening setting a hotprop.