This branch implements support of btree indexes instead of .kvi files.
Kvi also created, but not used during search/lookup, only for size
comparation.
Btree allows to make seek calls, while recsplit indexes does not support
that query.
During initial tests, btree indexes are ~1-3% smaller than recsplit but
avg lookup time bigger up to 1.5us (which compensates with ability to
fetch next key).
BtAlloc
```
337000 searches, last took 5.417µs avg=4.385µs next_access_last[of 10000 keys] 105ns
338000 searches, last took 3.792µs avg=4.385µs next_access_last[of 10000 keys] 103ns
339000 searches, last took 5.125µs avg=4.386µs next_access_last[of 10000 keys] 103ns
340000 searches, last took 4.75µs avg=4.386µs next_access_last[of 10000 keys] 103ns
341000 searches, last took 3.875µs avg=4.386µs next_access_last[of 10000 keys] 104ns
342000 searches, last took 6.042µs avg=4.387µs next_access_last[of 10000 keys] 107ns
343000 searches, last took 3.209µs avg=4.388µs next_access_last[of 10000 keys] 108ns
344000 searches, last took 2.292µs avg=4.393µs next_access_last[of 10000 keys] 113ns
345000 searches, last took 2.417µs avg=4.394µs next_access_last[of 10000 keys] 106ns
346000 searches, last took 2.625µs avg=4.394µs next_access_last[of 10000 keys] 103ns
347000 searches, last took 4.417µs avg=4.397µs next_access_last[of 10000 keys] 104ns
348000 searches, last took 5.458µs avg=4.398µs next_access_last[of 10000 keys] 103ns
349000 searches, last took 4.875µs avg=4.4µs next_access_last[of 10000 keys] 102ns
350000 searches, last took 4.25µs avg=4.4µs next_access_last[of 10000 keys] 104ns
351000 searches, last took 6.25µs avg=4.403µs next_access_last[of 10000 keys] 106ns
352000 searches, last took 3.125µs avg=4.404µs next_access_last[of 10000 keys] 106ns
```
Recsplit
```
354000 searches, last took 1.209µs avg=3.081µs
355000 searches, last took 1.333µs avg=3.076µs
356000 searches, last took 1.541µs avg=3.071µs
357000 searches, last took 1.333µs avg=3.066µs
358000 searches, last took 1.333µs avg=3.062µs
359000 searches, last took 1.167µs avg=3.057µs
360000 searches, last took 1.791µs avg=3.052µs
361000 searches, last took 1.209µs avg=3.048µs
362000 searches, last took 1.375µs avg=3.043µs
363000 searches, last took 1.166µs avg=3.038µs
364000 searches, last took 1.625µs avg=3.033µs
365000 searches, last took 1.125µs avg=3.029µs
366000 searches, last took 1.208µs avg=3.024µs
367000 searches, last took 1.333µs avg=3.019µs
368000 searches, last took 1.791µs avg=3.015µs
369000 searches, last took 1.333µs avg=3.01µs
370000 searches, last took 1.166µs avg=3.006µs
371000 searches, last took 1.125µs avg=3.001µs
372000 searches, last took 1.333µs avg=2.997µs
```
https://github.com/ledgerwatch/erigon-lib/pull/891/files
