Check for entities already processed when preparing data for FCV

This is a huge help on a Calendar View calendar. On a site we're trying to launch, it was taking 45-60 seconds to render the next month of the calendar.

This is with around 4,000 events in the database, many of them recurring.

Using a profiler, I found the Drupal\smart_date\Plugin\Field\FieldFormatter\SmartDateFormatterBase::formatSmartDate method was called nearly 1/2 million times!

After this patch, it's now getting called ~17K times, and it's down to ~8 seconds to move a month. Still could use improvement, but this was about an 80% improvement!

Thanks for the feedback! Merged the changes into the 4.0.x branch.

Automatically closed - issue fixed for 2 weeks with no activity.