add APIs necessary for log compaction algorithms

[staltz]

Context: https://github.com/ssb-ngi-pointer/jitdb/issues/199

Problem 1: we need to know when is indexing ongoing and/or when is querying ongoing, because we don't want compact to happen in parallel with those.

Problem 2: if compaction is ongoing, we can't allow JITDB queries to start.

Problem 3: reindex has to rebuild also the core indexes.

Solution:

• Introduces the public APIs indexingActive and queriesActive which are Obz's that emit numbers of how many are active at the moment.
• Monitor the status of compaction and guard the queries to wait for compaction to end first
• reindex rebuilds also the core indexes

These changes are already used and tested in https://github.com/ssb-ngi-pointer/ssb-db2/pull/339

[github-actions[bot]]

Benchmark results

Part	Speed	Heap Change	Samples
Count 1 big index (3rd run)	0.38ms ± 0.03ms	-1.8 kB ± 10.57 kB	59
Create an index twice concurrently	500.15ms ± 2.67ms	18.21 kB ± 25.39 kB	109
Load core indexes	0.8ms ± 0.01ms	92.7 B ± 181.62 B	9685
Load two indexes concurrently	407.67ms ± 2.56ms	-83.75 kB ± 106.33 kB	24
Paginate 10 results	26.35ms ± 1.64ms	11.51 kB ± 16.76 kB	29
Paginate 20000 msgs with pageSize=5	6103.73ms ± 86.69ms	145.33 kB ± 973.87 kB	5
Paginate 20000 msgs with pageSize=500	480.78ms ± 2.46ms	231.13 kB ± 415.86 kB	23
Query 1 big index (1st run)	742.65ms ± 11.93ms	-2.83 kB ± 38.49 kB	74
Query 1 big index (2nd run)	251.61ms ± 0.62ms	1.71 kB ± 10.18 kB	59
Query 3 indexes (1st run)	1068.12ms ± 14.93ms	-52.37 kB ± 150.24 kB	50
Query 3 indexes (2nd run)	218.64ms ± 1.08ms	21.41 kB ± 24.41 kB	62
Query a prefix map (1st run)	225.85ms ± 1.94ms	28.97 kB ± 534.77 kB	26
Query a prefix map (2nd run)	14ms ± 0.7ms	-12.99 kB ± 45.61 kB	26