Allow validators to serialize BL indexes to disk #4698

Job	Run time
fmt	15s
cackle	1m 32s
cargo-deny (advisories)	44s
build (gcc, current)	56m 14s
cargo-deny (bans licenses sources)	48s
build (gcc, current, --check-test-tx-meta)	55m 7s
rust-check-git-rev-deps	20s
build (gcc, next)	56m 24s
build (gcc, next, --check-test-tx-meta)	56m 37s
build (clang, current)	54m 0s
build (clang, current, --check-test-tx-meta)	50m 13s
build (clang, next)	57m 13s
build (clang, next, --check-test-tx-meta)	47m 0s
complete	1s
	7h 16m 28s

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