save work-in-progress indexes every 60s

[staltz]

This PR is now pending on ssbc/async-append-only-log#24 getting merged first and released.

[github-actions]

Benchmark results

Part	Duration
Load core indexes	9ms
Query 1 big index (1st run)	1038ms
Query 1 big index (2nd run)	339ms
Count 1 big index (3rd run)	1ms
Create an index twice concurrently	339ms
Query 3 indexes (1st run)	586ms
Query 3 indexes (2nd run)	284ms
Load two indexes concurrently	446ms
Paginate 1 big index	274ms
Query a prefix map (1st run)	276ms
Query a prefix map (2nd run)	6ms

[github-actions]

Benchmark results

Part	Duration
Load core indexes	21ms
Query 1 big index (1st run)	1071ms
Query 1 big index (2nd run)	308ms
Count 1 big index (3rd run)	1ms
Create an index twice concurrently	336ms
Query 3 indexes (1st run)	619ms
Query 3 indexes (2nd run)	284ms
Load two indexes concurrently	496ms
Paginate 1 big index	271ms
Query a prefix map (1st run)	305ms
Query a prefix map (2nd run)	9ms

[github-actions]

Benchmark results

Part	Duration
Load core indexes	8ms
Query 1 big index (1st run)	1092ms
Query 1 big index (2nd run)	307ms
Count 1 big index (3rd run)	1ms
Create an index twice concurrently	379ms
Query 3 indexes (1st run)	607ms
Query 3 indexes (2nd run)	283ms
Load two indexes concurrently	472ms
Paginate 1 big index	288ms
Query a prefix map (1st run)	299ms
Query a prefix map (2nd run)	5ms