Force fetching ep_wpcli_sync_interrupted transient from remote to allow for more reliable remote interruption

[rinatkhaziev]

Description

During some testing, it was discovered that wp vip-search stop-indexing doesn't reliably terminate the execution of an already-running process.

I think this is due to the cache value being stuck in local memory, this should help to prevent that by always forcing the remote fetch from the object cache backend.

Checklist

Please make sure the items below have been covered before requesting a review:

• This change works and has been tested locally (or has an appropriate fallback).
• This change works and has been tested on a Go sandbox.
• This change has relevant unit tests (if applicable).
• This change has relevant documentation additions / updates (if applicable).

Steps to Test

1. Check out PR.
2. Spin up an indexing process in a memcached-enabled environment
3. Run wp vip-search stop-indexing from another session
4. Repeat the process and verify it gets interrupted properly.

[rebeccahum]

Haven't tested extensively but this change makes sense to me.

[rebeccahum]

Opened upstream: 10up#2433