TailInitializationFilter that uses a specific timestamp/sequence number, not a time period

[rbasralian]

For the TailInitializationFilter, it would be more helpful in many cases to take the tail of the table starting at a specific timestamp, rather than starting at some period from the end of the table.

For example, if a DerivedTableWriter that writes a downsampled table restarts, it can fairly easily determine the earliest sequence number it needs from the source tables to rebuild its state up to the point that it stopped.

(Also, we should wrap it for Python.)