Draft: KV DB abstraction 2: electric boogaloo

Fixes #39.

Feature parity with KvTree:

• KvTree::get -> Map::get
• KvTree::insert -> Map::set
• KvTree::insert_batch
• KvTree::remove -> Map::del
• KvTree::iter
• KvTree::iter_from
• KvTree::increment -> 🗑
  • We desperately need to redo our PostgreSQL sequence approximation, the existing code scares me
  • Only 2 column families actually use the increment function, this can be implemented on top of the new abstraction instead of having a dedicated method
• KvTree::increment_batch -> 🗑
  • We desperately need to redo our PostgreSQL sequence approximation, the existing code scares me
  • Only 2 column families actually use the increment_batch function, this can be implemented on top of the new abstraction instead of having a dedicated method
• KvTree::scan_prefix -> Map::scan_prefix
• KvTree::watch_prefix
• KvTree::clear

Other things:

• Implement Map for RocksDB
• Implement Map for SQLite
• Replace at least one use of KvTree with Map
  • More replacements could be done incrementally