add compactionProgress() source API

[staltz]

Just bubbling up an AAOL API for ssb-db2 users (via muxrpc).

[arj03]

👍

[github-actions]

Benchmark results

Part	Duration
add 1000 elements	519ms
add 1000 private box1 elements	1157ms
unbox 1000 private box1 elements first run	123ms
unbox 1000 private box1 elements second run	92ms
add 1000 private box1 elements	1031ms
query 1000 elements first run	40ms
query 1000 elements second run	11ms
add 1000 private box2 elements	563ms
unbox 1000 private box2 elements first run	362ms
unbox 1000 private box2 elements second run	351ms
Migrate (+db1)	11334ms
Migrate (alone)	3916ms
Migrate (+db1 +db2)	8054ms
Migrate (+db2)	6374ms
Migrate continuation (+db2)	960ms
Memory usage without indexes	747.16 MB = 37.33 MB + etc
Initial indexing	689ms
Initial indexing maxCpu=86	4275ms
Initial indexing compat	622ms
Two indexes updating concurrently	961ms
key one initial	46ms
key two	1ms
key one again	1ms
reboot and key one again	46ms
latest root posts	770ms
latest posts	10ms
votes one initial	540ms
votes again	1ms
hasRoot	390ms
hasRoot again	0ms
author one posts	454ms
author two posts	23ms
dedicated author one posts	478ms
dedicated author one posts again	0ms
Maximum memory usage	1010.19 MB = 58.80 MB + etc
Indexes folder size	9.97mb

[github-actions]

Benchmark results

Part	Duration
add 1000 elements	485ms
add 1000 private box1 elements	1151ms
unbox 1000 private box1 elements first run	119ms
unbox 1000 private box1 elements second run	98ms
add 1000 private box1 elements	1136ms
query 1000 elements first run	41ms
query 1000 elements second run	14ms
add 1000 private box2 elements	619ms
unbox 1000 private box2 elements first run	361ms
unbox 1000 private box2 elements second run	352ms
Migrate (+db1)	11114ms
Migrate (alone)	3798ms
Migrate (+db1 +db2)	8034ms
Migrate (+db2)	6170ms
Migrate continuation (+db2)	871ms
Memory usage without indexes	740.55 MB = 32.82 MB + etc
Initial indexing	730ms
Initial indexing maxCpu=86	4309ms
Initial indexing compat	605ms
Two indexes updating concurrently	956ms
key one initial	49ms
key two	1ms
key one again	0ms
reboot and key one again	42ms
latest root posts	779ms
latest posts	11ms
votes one initial	549ms
votes again	1ms
hasRoot	375ms
hasRoot again	1ms
author one posts	450ms
author two posts	20ms
dedicated author one posts	476ms
dedicated author one posts again	1ms
Maximum memory usage	993.53 MB = 57.23 MB + etc
Indexes folder size	9.97mb