Make it possible to throttle (Tiered)MergePolicy when commit rate is high

As discussed in a recent mailing list thread [1] and observed in a project using Lucene (see OAK-5192 and OAK-6710), it is sometimes helpful to throttle the aggressiveness of (Tiered)MergePolicy when commit rate is high.

In the case of Apache Jackrabbit Oak a dedicated MergePolicy was implemented [2].

That MP doesn't merge in case the number of segments is below a certain threshold (e.g. 30) and commit rate (docs per sec and MB per sec) is high (e.g. above 1000 doc / sec , 5MB / sec).

In such impl, the commit rate thresholds adapt to average commit rate by means of single exponential smoothing.

The results in that specific case looked encouraging as it brought a 5% perf improvement in querying and ~10% reduced IO. However Oak has some specifics which might not fit in other scenarios. Anyway it could be interesting to see how this behaves in plain Lucene scenario.

[1] : http://markmail.org/message/re3ifmq2664bqfjk

[2] : https://github.com/apache/jackrabbit-oak/blob/trunk/oak-lucene/src/main/java/org/apache/jackrabbit/oak/plugins/index/lucene/writer/CommitMitigatingTieredMergePolicy.java