Add thread safe storage credential update

Author: will-tong

Microsoft.WindowsAzure.Storage/includes/was/service_client.h

@@ -74,11 +74,22 @@ namespace azure { namespace storage {
         /// Gets the storage account credentials for the service client.
         /// </summary>
         /// <returns>The storage account credentials for the service client.</returns>
-        const azure::storage::storage_credentials& credentials() const
+        const azure::storage::storage_credentials& credentials()
         {
+            pplx::extensibility::scoped_read_lock_t guard(m_mutex);
             return m_credentials;
         }
 
+        /// <summary>
+        /// Sets the storage credentials to use for the service client.
+        /// </summary>
+        /// <param name="credentials">The <see cref="azure::storage::storage_credentials" /> to use.</param>
+        void set_storage_credentials(azure::storage::storage_credentials credentials)
+        {
+            pplx::extensibility::scoped_rw_lock_t guard(m_mutex);
+            m_credentials = std::move(credentials);
+        }
+
         /// <summary>
         /// Gets the authentication scheme to use to sign HTTP requests for the service client.
         /// </summary>
@@ -150,6 +161,7 @@ namespace azure { namespace storage {
 
     private:
 
+        pplx::extensibility::reader_writer_lock_t m_mutex;
         storage_uri m_base_uri;
         azure::storage::storage_credentials m_credentials;
         azure::storage::authentication_scheme m_authentication_scheme;