Add better graph stats outputs & new input option

[benwtrent]

This adds more graph stats outputs for HNSW.

• Adds connectiveness percentage for each layer starting from the graph entry point
• Adds histogram output for number of connections for each layer (not just the bottom)

Additional adjust the following:

• Adds new randomCommits input for aggressively testing merging by randomly calling iw.commit()
• Allows for forceMerge to run even without reindex. This is helpful for benchmarking & testing the differences between multiple segments and then force-merging them to a single graph.

[benwtrent]

Related to: https://github.com/mikemccand/luceneutil/pull/236

But only does the by layer connectiveness tests, seems like a good enough test to me.

[benwtrent]

Here is an example printout for a single graph:

Graph level=4 size=2, Fanout min=1, mean=1.00, max=1
%   0  10  20  30  40  50  60  70  80  90 100
    0   1   1   1   1   1   1   1   1   1   1   1   1   1   1
Graph level=3 size=19, Fanout min=4, mean=6.00, max=9
%   0  10  20  30  40  50  60  70  80  90 100
    0   4   4   5   5   5   6   6   8   8   9
Graph level=2 size=334, Fanout min=12, mean=15.88, max=16
%   0  10  20  30  40  50  60  70  80  90 100
    0  16  16  16  16  16  16  16  16  16  16
Graph level=1 size=5548, Fanout min=16, mean=16.00, max=16
%   0  10  20  30  40  50  60  70  80  90 100
    0  16  16  16  16  16  16  16  16  16  16
Graph level=0 size=87000, Fanout min=6, mean=26.72, max=32
%   0  10  20  30  40  50  60  70  80  90 100
    0  16  20  23  27  32  32  32  32  32  32
Graph level=4 size=2, connectedness=1.00
Graph level=3 size=19, connectedness=1.00
Graph level=2 size=334, connectedness=1.00
Graph level=1 size=5548, connectedness=1.00
Graph level=0 size=87000, connectedness=1.00

[mikemccand]

Cool, thanks @benwtrent! I think we should merge https://github.com/mikemccand/luceneutil/pull/236 too? It's draft now, I'm not sure why ... progress not perfection!