Refactor session management to use a new "Agent" API (#165)

* Add the atp-agent implementation (temporarily in this repo)

* Rewrite all session & API management to use the new atp-agent

* Update tests for the atp-agent refactor

* Refactor management of session-related state. Includes:
- More careful management of when state is cleared or fetched
- Debug logging to help trace future issues
- Clearer APIs overall

* Bubble session-expiration events to the user and display a toast to explain

* Switch to the new @atproto/api@0.1.0

__mocks__/state-mock.ts

@@ -648,8 +648,8 @@ export const mockedRootStore = {
   resolveName: jest.fn(),
   serialize: jest.fn(),
   hydrate: jest.fn(),
-  fetchStateUpdate: jest.fn(),
-  clearAll: jest.fn(),
+  updateSessionState: jest.fn(),
+  clearAllSessionState: jest.fn(),
   session: mockedSessionStore,
   nav: mockedNavigationStore,
   shell: mockedShellStore,