change default to sync, minus persistent exts

src/Lean/Environment.lean

@@ -869,7 +869,7 @@ end EnvExtension
    Note that by default, extension state is *not* stored in .olean files and will not propagate across `import`s.
    For that, you need to register a persistent environment extension. -/
 def registerEnvExtension {σ : Type} (mkInitial : IO σ)
-    (asyncMode : EnvExtension.AsyncMode := .mainOnly) : IO (EnvExtension σ) := do
+    (asyncMode : EnvExtension.AsyncMode := .sync) : IO (EnvExtension σ) := do
   unless (← initializing) do
     throw (IO.userError "failed to register environment, extensions can only be registered during initialization")
   let exts ← EnvExtension.envExtensionsRef.get