fix: make storage provider & initialize synchronous (#305)

openAI reported:


The Statsig.start() API previously made Statsig available to query
feature flags as soon as it returned (because the client variable is set
synchronously).
However, when passing a storageProvider object, it now performs a
DispatchQueue.main.async call before setting client. This means that all
the checkGate() calls we make immediately after result in seeing this in
the console:
`[Statsig]: Must start Statsig first and wait for it to complete before
calling checkGate. Returning false as the default.`

I don't see a reason for this DispatchQueue.main.async . Could this be
changed so initialization is consistent whether passing storageProvider
or not?

Sources/Statsig/Statsig.swift

@@ -43,10 +43,8 @@ public class Statsig {
                 _initialize()
             }
         } else if let storageProvider = options?.storageProvider {
-            DispatchQueue.main.async {
-                StatsigUserDefaults.defaults = StorageProviderBasedUserDefaults(storageProvider: storageProvider)
-                _initialize()
-            }
+            StatsigUserDefaults.defaults = StorageProviderBasedUserDefaults(storageProvider: storageProvider)
+            _initialize()
         } else {
             _initialize()
         }