change default to sync

src/Lean/Environment.lean

@@ -865,7 +865,7 @@ 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
@@ -954,7 +954,7 @@ structure PersistentEnvExtension (α : Type) (β : Type) (σ : Type) where
   addEntryFn      : σ → β → σ
   exportEntriesFn : σ → Array α
   statsFn         : σ → Format
-  asyncMode       : EnvExtension.AsyncMode := .mainOnly
+  asyncMode       : EnvExtension.AsyncMode := .sync
 
 instance {α σ} [Inhabited σ] : Inhabited (PersistentEnvExtensionState α σ) :=
   ⟨{importedEntries := #[], state := default }⟩