APIs to reset TimePoint

[jleibs]

Calls to set_time_sequence / set_time_seconds currently mutate global (thread-local) state. While this is generally convenient, in a jupyter context it means that timeline data is likely to "leak" between two cells.

At a minimum we probably want an API to clear this state, though it's not obvious what to call it. Maybe: rr.reset_timepoint()?

A related question is whether we should always do this as part of rr.init(...).

[jleibs]

Resolved in: #1826