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

__tests__/state/models/root-store.test.ts

@@ -17,7 +17,7 @@ describe('rootStore', () => {
   })
 
   it('should call the clearAll() resets state correctly', () => {
-    rootStore.clearAll()
+    rootStore.clearAllSessionState()
 
     expect(rootStore.session.data).toEqual(null)
     expect(rootStore.nav.tabs).toEqual([