Migrate high-volume DBs to LMDB

Databases that need to store a large volume (blockchain and indexes in particular) must be migrated to LMDB.

Use of kv crate: https://docs.rs/kv

changed milestone to %v0.3 Minimal mirror node

added D-hard S-to do in priority labels

created merge request !193 (merged) to address this issue

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closed via merge request !193 (merged)