Disable Jetpack Sync during Shutdown for REST API Requests

[allendav]

Jetpack sync during the shutdown hook is adding about 4 seconds to product creation and probably other requests too.

Let's filter jetpack_sync_sender_should_load to false during a REST_REQUEST as a temporary measure, until

• WordPress core has a release that includes a fix for https://core.trac.wordpress.org/ticket/41358#ticket
• Jetpack has a release that includes Sync: Add Async Sender Automattic/jetpack#7482

See also woocommerce/woocommerce#16158

cc @mikejolley @westi @DanReyLop @justinshreve