Revert "GULMutableDictionary: use dispatch_sync instead of async." (#3337)

* Revert "GULMutableDictionary: use `dispatch_sync` instead of `async`. (#3322)"

This reverts commit f7c88f1.

* GoogleUtilities 6.2.3

Author: maksymmalyhin

GoogleUtilities.podspec

@@ -1,6 +1,6 @@
 Pod::Spec.new do |s|
   s.name             = 'GoogleUtilities'
-  s.version          = '6.2.2'
+  s.version          = '6.2.3'
   s.summary          = 'Google Utilities for iOS (plus community support for macOS and tvOS)'
 
   s.description      = <<-DESC

GoogleUtilities/CHANGELOG.md

@@ -1,3 +1,6 @@
+# 6.2.3
+- Revert "Fix `GULMutableDictionary` thread-safety." (#3322)
+
 # 6.2.2
 - Add explicit Foundation import for headers.
 - Fix headers import. (#3277)

GoogleUtilities/Network/GULMutableDictionary.m

@@ -51,19 +51,19 @@ - (id)objectForKey:(id)key {
 }
 
 - (void)setObject:(id)object forKey:(id<NSCopying>)key {
-  dispatch_sync(_queue, ^{
+  dispatch_async(_queue, ^{
     self->_objects[key] = object;
   });
 }
 
 - (void)removeObjectForKey:(id)key {
-  dispatch_sync(_queue, ^{
+  dispatch_async(_queue, ^{
     [self->_objects removeObjectForKey:key];
   });
 }
 
 - (void)removeAllObjects {
-  dispatch_sync(_queue, ^{
+  dispatch_async(_queue, ^{
     [self->_objects removeAllObjects];
   });
 }