Activity additional info

[bergundy]

There's a couple of extra fields to help understand the status of a pending activity:

1. Add last_failure_time to PendingActivityInfo. Populated once an attempt failure is recorded for any reason that an attempt might fail, including any form of timeout.
2. Add next_attempt_schedule_time to PendingActivityInfo. Shown if activity is in backoff state (waiting for retry).

[ychebotarev]

3. Add next_retry_interval