fix: ensure we don't send multiple flag requests in parallel

[frankh]

Problem

currently if we call e.g. getFeatureFlags() 100 times straight away after initialisation we will send 100 fetch requests before the first one returns

this is even worse if e.g. we are erroring so never setting loadedSuccessfullyOnce

Changes

add logic to check if we have an inflight request already and block on the existing one instead of starting a new one

Release info Sub-libraries affected

Bump level

• [ ] Major
• [ ] Minor
• [x] Patch

Libraries affected

• [ ] All of them
• [ ] posthog-web
• [x] posthog-node
• [ ] posthog-ai
• [ ] posthog-react-native

Changelog notes

• Fix: don't send multiple feature flag fetch requests in parallel.

[marandaneto]

@PostHog/team-feature-flags should this be finished/merged/released?

[marandaneto]

moving to draft until we hear from @PostHog/team-feature-flags