NewInMemoryDatabase returns a new instance of the in-memory database.

InMemoryDatabase is a simgple in-memory storage system for local prototyping, development, and testing.

Note represents a complete note, e.g., as stored in the database.

PartialNote represents a note as may be sent by a user when creating a note, e.g., missing an Id field.

Database represents a generic abstract storage system, with several potential implementations.