Add indexPctDeletedTarget as a parameter to TieredMergePolicy to control more aggressive merging

Spinoff of LUCENE-7976 to keep the two issues separate.

The current TMP allows up to 50% deleted docs, which can be wasteful on large indexes. This parameter will do more aggressive merging of segments with deleted documents when the total percentage of deleted docs in the entire index exceeds it.

Setting this to 50% should approximate current behavior. Setting it to 20% caused the first cut at this to increase I/O roughly 10%. Setting it to 10% caused about a 50% increase in I/O.

I was conflating the two issues, so I'll change 7976 and comment out the bits that reference this new parameter. After it's checked in we can bring this back. That should be less work than reconstructing this later.

Among the questions to be answered:

1> what should the default be? I propose 20% as it results in significantly less space wasted and helps control heap usage for a modest increase in I/O.

2> what should the floor be? I propose 10% with strong documentation warnings about not setting it below 20%.

3> should there be two parameters? I think this was discussed somewhat in 7976. The first cut at this used this number for two purposes:
3a> the total percentage of deleted docs index-wide to trip this trigger
3b> the percentage of an individual segment that had to be deleted if the segment was over maxSegmentSize/2 bytes in order to be eligible for merging. Empirically, using the same percentage for both caused the merging to hover around the value specified for this parameter.

My proposal for <3> would be to have the parameter do double-duty. Assuming my preliminary results hold, you specify this parameter at, say, 20% and once the index hits that % deleted docs it hovers right around there, even if you've forceMerged earlier down to 1 segment. This seems in line with what I'd expect and adding another parameter seems excessively complicated to no good purpose. We could always add something like that later if we wanted.