Add flag to disable mmap in all sql connection

sql/connection.h

Index: sql/connection.h
diff --git a/sql/connection.h b/sql/connection.h
index b35e2faaf32c34ca6775df1cd5723eb2e66e6c88..4606a1f357c3ae00c0aee789af5dc91487294d99 100644
--- a/sql/connection.h
+++ b/sql/connection.h
@@ -149,9 +149,12 @@ class SQL_EXPORT Connection {
   // other platforms.
   void set_restrict_to_user() { restrict_to_user_ = true; }
 
-  // Call to opt out of memory-mapped file I/O.
+  // Call to opt out of memory-mapped file I/O on per connection basis.
   void set_mmap_disabled() { mmap_disabled_ = true; }
 
+  // Call to opt out of memory-mapped file I/O on all connections.
+  static void set_mmap_disabled_by_default();
+
   // Set an error-handling callback.  On errors, the error number (and
   // statement, if available) will be passed to the callback.
   //