Fully clear queued updates before adding a new computed column

[sc1f]

When a computed column is added, a new Table is created with the computed column, and Perspective internally invalidates and cleans up the old Table.

However, if updates were queued on the old Table and then called for process, the state between the old Table and the new Table (both of which have the computed column) may fall out of sync and cause errors at random.

This PR adds a call to call_process before creating the computed column, ensuring that all queued updates are processed before the old Table is invalidated.

It also reverses a change on the streaming.html example, by reducing the tick rate to something approaching sanity and adding a date column for full type coverage.

[texodus]

Looks good! Thanks for the PR!

[texodus]

Fixes #972