Implement notification-based sync from Exchange

[juyrjola]

The customer requires as small latency as possible when syncing reservations from Exchange to Respa. The customer needs this because the customer has learned that small numbers are good.

It might be implemented through the Steaming Notification system in EWS. Exchange has some limitations in the number of concurrent connections, so several subscriptions should probably be listened to with the same TCP connection (https://msdn.microsoft.com/en-us/library/office/dn458789(v=exchg.150).aspx).

The sync code should handle:

• Meeting created / modified / deleted.
• Possible out-of-sync scenarios that result in the TCP connection dying unexpectedly.

[akx]

I respectfully disagree with the "because" reasoning here. ;)