- Remove a few unnecessary dependencies
- Add some comments around usage of 'api'
- Mark some of the dependencies as 'api' if they're necessary for use of the module
A storage implementation might need to cleanup its allocated resources,
and this provides an entry point for that to happen.
An example might be closing a native database connection that's maintained
by the Rust Places library.
Underlying HistoryStorage implementations might be blocking on IO (Rust Places, room),
or migth be non-blocking (in-memory). A Deferred return type for get* methods makes
it convenient to wrap both regular and `async` operations. Use of 'suspend' functions
serves the same role for write* methods.
This change makes it obvious at the engine call-site that the underlying implementation
might suspend, at which point we can wrap calls in `runBlocking` or use another coroutine builder.
