Issue997 - Eliminate retry file race condition and make AM work more like a sarracenia flow

[andreleblanc11]

Solves #997

[petersilva]

Looks good!

[gcglinton]

This broke the retry unit tests, because the driver selection got moved from __init__ into on_start, so now none of the test logic works.

In all the failing tests of retry_test.py, it should be enough to call retry_disk.on_start() or retry_redis.on_start() right after those retry objects (retry_disk, retry_redis) are instantiated. Like right after lines 75, 82, 109, 115, etc..

[petersilva]

yes the I found the same issue, there is a a commit 25df985
It is in the other PR ( #1003 )