Periodically refresh credentials

[devigned]

Currently, there is no background thread renegotiating credentials periodically. As a result, the link will periodically die due to an expiration of creds. This PR is intended to fix this issue.

#116

[devigned]

Based on the endurance test below, the periodic expiring of credentials which causes the link to be terminated has been fixed.

davidjustice at extremis in ~/code/devigned/testbus on master [!]
$ ./bin/testbus sparse-test --conn-str 'my-connection-string'
INFO[2019-07-17 13:30:55] Sent: "185801b2-4d56-4284-9e75-cd0f55237536"
INFO[2019-07-17 13:30:55] next send at: "2019-07-17 13:55:36"
INFO[2019-07-17 13:30:55] Received: "185801b2-4d56-4284-9e75-cd0f55237536"
INFO[2019-07-17 13:55:41] Sent: "8975f27d-ea29-45f6-91b9-1bc82b381262"
INFO[2019-07-17 13:55:41] Sent: "f56faf41-eb03-4e0b-ad68-3d18c55b8be3"
INFO[2019-07-17 13:55:41] next send at: "2019-07-17 13:56:28"
INFO[2019-07-17 13:55:41] Received: "8975f27d-ea29-45f6-91b9-1bc82b381262"
INFO[2019-07-17 13:55:41] Received: "f56faf41-eb03-4e0b-ad68-3d18c55b8be3"
INFO[2019-07-17 13:56:28] Sent: "edfb7714-389c-477d-81ba-e1fd79d35c33"
INFO[2019-07-17 13:56:28] Sent: "561c6a27-154d-4e6f-926d-c9587d32fd1e"
...
INFO[2019-07-18 02:11:14] Sent: "d1bfa589-3474-466c-81d7-1cc92c242abb"
INFO[2019-07-18 02:11:14] Sent: "111f05f4-5d28-4309-93dd-2710a4e4971e"
INFO[2019-07-18 02:11:14] next send at: "2019-07-18 02:36:51"
INFO[2019-07-18 02:11:14] Received: "d1bfa589-3474-466c-81d7-1cc92c242abb"
INFO[2019-07-18 02:11:14] Received: "111f05f4-5d28-4309-93dd-2710a4e4971e"
INFO[2019-07-18 02:36:55] Sent: "cc76628a-ba22-4270-8dd1-ebe5ed9a708a"
INFO[2019-07-18 02:36:55] Sent: "27b0573c-0d13-4b18-9fda-c7e63773495a"
INFO[2019-07-18 02:36:55] next send at: "2019-07-18 02:47:36"
INFO[2019-07-18 02:36:55] Received: "cc76628a-ba22-4270-8dd1-ebe5ed9a708a"
INFO[2019-07-18 02:36:55] Received: "27b0573c-0d13-4b18-9fda-c7e63773495a"
INFO[2019-07-18 02:47:40] Sent: "7e8180cb-5d4a-42f3-975c-7633be52050c"
INFO[2019-07-18 02:47:40] Sent: "6d2b96a3-4983-46bb-8f7d-31da047cd738"
INFO[2019-07-18 02:47:40] next send at: "2019-07-18 03:11:33"
INFO[2019-07-18 02:47:40] Received: "7e8180cb-5d4a-42f3-975c-7633be52050c"
INFO[2019-07-18 02:47:40] Received: "6d2b96a3-4983-46bb-8f7d-31da047cd738"
INFO[2019-07-18 03:11:43] Sent: "08d01c4f-efdc-4790-a844-b52b3a2ce368"
INFO[2019-07-18 03:11:43] Sent: "af29c3eb-2020-4ac1-acab-0e2d28e1ccdc"
INFO[2019-07-18 03:11:43] next send at: "2019-07-18 03:39:54"
INFO[2019-07-18 03:11:43] Received: "08d01c4f-efdc-4790-a844-b52b3a2ce368"
INFO[2019-07-18 03:11:43] Received: "af29c3eb-2020-4ac1-acab-0e2d28e1ccdc"
...