Event replay and cleanup

This commit introduces the ability to reconfigure modules/event subscribers by
replaying previously stored events at startup. This way modules/subscribers can
always come up with the latest activity and data that was present at close at the
last run of the program. This process also includes the disposal of unneeded events
to minimize disk use over time. The changes include the following:

1. Event interface is extended with the Disposable() function
   unimplemented stubs for existing interface implementations
2. state.Init() is split into Init() and Start()
   Init() now initialized the file used by eventStream, and puts in place a 0 size
   memory cache as a temporary measure on top of the file.
   Start() reads Pop()s documents one by one from the temp nil-cache eventStream,
   it attempts to dispose of each event and if the event is not disposable it is
   pushed onto a second temporary no-memory-cache buffer which is then reverse ordered.
   The underlying file is truncated and reopened.
   Finally, the real eventStream is allocated, with in memory cache. All events in the
   second buffer are published (sent to subscribers as well as added to the new eventStream).
3. Updates to main() to support Init() vs Start()

Signed-off-by: Ava Affine <ava@sunnypup.io>

main.go

@@ -32,10 +32,15 @@ func main() {
 	); err != nil {
 		log.Fatalf("couldn't initialize state engine: %s", err.Error())
 	}
-	defer state.Teardown()
 
+	// TODO: start modules HERE and not elsewhere
 	err = startBot()
 
+	if err := state.Start(); err != nil {
+		log.Fatal("failed to start state machine: %s", err.Error())
+	}
+	defer state.Teardown()
+
 	if err != nil {
 		log.Fatal(err)
 	}