set_roll_back should only rollback initialized connections

The exception handler call "connections.all()". This creates a database connection to all defined databases. 
As a small optimization you can use "connections.all(initialized_only=True)" to rollback only the database to which the current thread has open connections.

(My application can have many databases defined, and this loop is identified as a source of many idle database connections)

Author: jdetaeye

rest_framework/views.py

@@ -64,7 +64,7 @@ def get_view_description(view, html=False):
 
 
 def set_rollback():
-    for db in connections.all():
+    for db in connections.all(initialized_only=True):
         if db.settings_dict['ATOMIC_REQUESTS'] and db.in_atomic_block:
             db.set_rollback(True)