The HMapDatabase mimics the behaviour of LMDBDatabase without keeping a persistent copy of the key-value records. It allows key-value pairs to be inserted, retrieved and removed in a thread-safe manner.
This is a simple wrapper struct that lifts the generic parameters so that KeyValStore can be implemented on LMDBDatabase. LMDBDatabase doesn't have the generics at the struct level because the LMDBStore can contain many instances of LMDBDatabase each with different K and V types.
Used to indicate whether an iteration should continue or break (i.e not called again)
General CRUD behaviour of Key-value store implementations.