Impossible to close the client with feature flag poller

[zaynetro]

Hi! It is impossible to close posthog.Client when feature flags are enabled and no requests to get all flags were made.

What happens:

1. FeatureFlagsPoller.run starts
2. Execution is blocked on poller.loaded <- false in fetchNewFeatureFlags because nobody reads from that channel
3. Infinite loop never starts so <-poller.shutdown is never consumed hence blocking Client.Close method here.
4. Things work if we GetFeatureFlags at least once. It starts reading poller.loaded channel here

Minimal reproduction example:

package main

import (
	"fmt"
	"os"

	"github.com/posthog/posthog-go"
)

func main() {
	client, err := posthog.NewWithConfig("123", posthog.Config{
		PersonalApiKey: "123",
	})
	if err != nil {
		fmt.Println("could not initialize posthog client", err)
		os.Exit(1)
	}

	// NOTE: Uncomment for things to start working
	// flags, err := client.GetFeatureFlags()
	// fmt.Println("Flags", flags, err)

	fmt.Println("Trying to close the client...")
	if err := client.Close(); err != nil {
		fmt.Println("Failed to close:", err)
	}

	fmt.Println("Client closed.")
}