SearchPlugins do not update all Channels

[michaelbromley]

Describe the bug
If a Product/ProductVariant is assigned to multiple channels, then updates to it (e.g. changing the name or other property) only triggers an update to the search index for the current Channel.

To Reproduce
Steps to reproduce the behavior:

In the Admin UI:

1. Create a second Channel
2. Add a product from the Default Channel to that second Channel
3. Switch to the second Channel and edit that product's name
4. Go back to the Product list. Note that the name change is reflected in the list
5. Switch to the Default Channel. Note that the product name has not been updated.

Expected behavior
When updating a Product or ProductVariant, the search index needs to be update for all Channels to which it is assigned. This includes changes caused by changes to Collection filters.

Environment (please complete the following information):

• @vendure/core version: 0.18.0
• Nodejs version any
• Database (mysql/postgres etc): any