Clean the post cache after using `$wpdb->update()`

On setups with a persistent object cache, the post cache needs to be cleared upon data alteration. Otherwise, future requests / transformations could use stale cache data.
Persist the current value for the `post_date` field when a post is updated.

In `wp_update_post()`, core will reset the `post_date` field if the post currently has a status of `draft`, `pending` or `auto-draft`.
The Edit Flow calendar uses the `post_date` field as the canonical reference for the post's position on the calendar.
To prevent the post from moving to the current date when it's updated, we need to persist the `post_date` value for the post.
Update readme with bugfix mention
Don't try to persist `post_date` if `post_date_gmt` is set.

`post_date_gmt` is how WordPress tracks whether a post is scheduled or not. If a post is scheduled, Edit Flow's calendar will handle the post differently.
Merge pull request #225 from Automattic/fix-calendar-post-date-225

Saving a change to a post removes its far-future position in the calendar