Despite the non-thread-safety of HyperLogLogCollector, it is actually currently used
by multiple threads during realtime indexing. HyperUniquesAggregator's "aggregate" and
"get" methods can be called simultaneously by OnheapIncrementalIndex, since its
"doAggregate" and "getMetricObjectValue" methods are not synchronized.

This means that the optimization of HyperLogLogCollector.fold in apache#3314 (saving and
restoring position rather than duplicating the storage buffer of the right-hand side)
could cause corruption in the face of concurrent writes.

This patch works around the issue by duplicating the storage buffer in "get" before
returning a collector. The returned collector still shares data with the original one,
but the situation is no worse than before apache#3314. In the future we may want to consider
making a thread safe version of HLLC that avoids these kinds of problems in realtime
indexing. But for now I thought it was best to do a small change that restored the old
behavior.