IOR does not handle events while doing initial sync

This issue was caught by a test failure:

• Test creates 100 gateways
• In its initial sync, IOR reads from the store, but it returns only 50 gateways.
• IOR starts creating the routes for those 50 gateways.
• At the same time, ADD events keep coming, for all 100 gateways, but IOR ignores them until initial sync finishes.
• After initial sync finishes, 50 routes are created and are "in sync" with the initial 50 gateways.
• Test expects 100 routes. FAIL.

The fact that the store returned only 50 gateways triggered a real issue: IOR should not ignore events while doing the initial sync. A possible solution would be to queue the events as they arrive and process them after the initial sync.