Speed up initial mib sync performance

Store the onu omci mib in memory only.
This speeds up onu activation considerably especially on hardware
constrained systems.

Future plans are to re-introduce mib persistence along with the
use of the 2.0 core and per adapter processes.

Change-Id: I9cdf11980533dde153d482e90c6fdfe4a9ce3408
diff --git a/voltha/extensions/omci/openomci_agent.py b/voltha/extensions/omci/openomci_agent.py
index bc84446..c955018 100644
--- a/voltha/extensions/omci/openomci_agent.py
+++ b/voltha/extensions/omci/openomci_agent.py
@@ -40,8 +40,8 @@
 OpenOmciAgentDefaults = {
     'mib-synchronizer': {
         'state-machine': MibSynchronizer,  # Implements the MIB synchronization state machine
-        # 'database': MibDbVolatileDict,     # Implements volatile ME MIB database
-        'database': MibDbExternal,         # Implements persistent ME MIB database
+        'database': MibDbVolatileDict,     # Implements volatile ME MIB database
+        # 'database': MibDbExternal,         # Implements persistent ME MIB database
         'advertise-events': True,          # Advertise events on OpenOMCI event bus
         'tasks': {
             'mib-upload': MibUploadTask,