Use same BM25 k1/b parameters across engines.

[jpountz]

The k1 and b parameters of BM25 can influence what hits may be dynamically pruned and thus performance numbers, so it would be good to use the same values across engines. Currently it looks like engines use their own defaults, which seem to be k1=0.9 and b=0.4 for PISA, and k1=1.2 and b=0.75 for Lucene and Tantivy.

[jpountz]

To get a sense of the influence of these parameters on query performance, I compared Lucene-9.8 with 1.2/0.75 against 0.9/0.4 on the TOP_100 command. I'm getting:

• 4.6% better latency on average for intersections with 0.9/0.4
• 4.2% better latency on average for unions with 0.9/0.4

So it's not huge but significant and extremely consistent:

• 7 queries get better latencies with 1.2/0.75
• 2 queries get the same latencies
• 893 queries get a better latency with 0.9/0.4