cot.verify should cache requests made to Github

[JohanLorenzo]

See #330 (review)

We should fix errors like:

2019-04-23T20:16:03    DEBUG -  GET https://github.com/mozilla/application-services/branch_commits/0a49320a24b2fc826f0153c0627baba1aa94d429
2019-04-23T20:16:03    DEBUG -  GET https://github.com/mozilla/application-services/branch_commits/0a49320a24b2fc826f0153c0627baba1aa94d429
2019-04-23T20:16:04    DEBUG -  GET https://github.com/mozilla/application-services/branch_commits/0a49320a24b2fc826f0153c0627baba1aa94d429
2019-04-23T20:16:04    DEBUG -  GET https://github.com/mozilla/application-services/branch_commits/0a49320a24b2fc826f0153c0627baba1aa94d429
2019-04-23T20:16:04    DEBUG -  Status 429
2019-04-23T20:16:05    DEBUG -  Status 429
2019-04-23T20:16:05    DEBUG -  Status 429
2019-04-23T20:16:05    DEBUG -  Status 429
2019-04-23T20:16:05    ERROR - Hit ScriptWorkerException: Bad status 429

One way to solve this is to cache the request made here

scriptworker/scriptworker/github.py

Line 117 in 30a54b0

html_data = await retry_request(context, url)

. In fact, we don't need to call this URL 4 times in a row.

[JohanLorenzo]

For the record, android-components saw about a dozen identical failures, yesterday. For instance: https://tools.taskcluster.net/groups/JpNiL1RTQau4tf_yYnMriA/tasks/Mj94usLBTo-vXnvL-gQ8YA/runs/1/logs/public%2Flogs%2Fchain_of_trust.log#L105