This also puts the entire state of `StripeBilling` behind a `RwLock`.
When fetching the existing prices and meters, or when inserting new
ones, we acquire a write lock and hold it until the Stripe request
completes. This prevents two concurrent calls to `get_or_insert_price`
from inserting the same data twice.
Creating a new meter/price is unusual, so in practice we'll acquire a
read lock most of the time.
/cc @rtfeldman @maxdeviant
Release Notes:
- N/A
Antonio Scandurra
6e2869a321