Khepri: high memory usage during import

[mkuratczyk]

NOTE: this issue is related to an unreleased feature

bazel run --config=local broker
bazel run --config=local rabbitmqctl enable_feature_flag raft_based_metadata_store_phase1
bazel run --config=local rabbitmqctl import_definitions q1-with-100k-topic-bindings.json

Memory usage goes up to almost 4GB:

For comparison, with Mnesia it doesn't go above 800MB:

[mkuratczyk]

I've re-tested this with the most recent changes. It's significantly better, with the maxium usage of ~1.4GB:

[mkuratczyk]

Restested a year later. The peak is below 900MB. Sill higher than Mnesia (600MB), but the import is significantly faster (90s for Khepri, 174s for Mnesia), so I think that's a decent trade-off.

Worth mentioning that the migration to khepri after importing this file took 14 minutes, which is something worth looking into, but this will be a separate issue.