Query planning cache warm up improvements

[Geal]

the warm_up_queries option takes the most used queries from the in memory cache, and creates query plans for them when a new schema comes in, before switching the router to this new schema. This makes schema updates much smoother, because it won't delay most of the queries coming, since they already have a query plan available.

It could benefit from a few improvements though:

• Add metrics tracking schema loading and query planner warm up time #3767
• Activate query planner warm up by default #3766
• Many Redis requests are timing out and throwing errors #3621
• plan queries in random order during warm up #3769
• add persisted queries to the query planner warm up #3770
• document the query plan cache warm up #3815

Follow up:

• Configurable delay between plans during warm up #3768
• Add a priority queue to query planning #3771

[Geal]

action: break this down in smaller tickets

[abernix]

I'm going to close this umbrella issue now since there are so few sub-tickets left that aren't completed. We can track the remaining two on their own.