R1:cbd95b91b5f1

Implement unsubscription in the Aphlict client.

Summary: Ref T5284. When an `AphlictClient` is closed, it will eventually be purged from the pool by the `AphlictMaster`. When this happens, also unsubscribe the purged client from all notifications, and send an `unsubscribe` command to the Aphlict server if possible.

Test Plan:
Verified the output of the Aphlict server (running in debug mode):

```
[Wed Jun 11 2014 23:21:31 GMT+0000 (UTC)] <FlashPolicy> Policy Request From ::ffff:192.168.1.1
[Wed Jun 11…