Modify approle tidy to validate dangling accessors

[jefferai]

No description provided.

[jefferai]

This fixes a race condition that can be found with the new test. It's difficult to trigger -- after a whole lot of tweaking, maybe 25% of my test runs were able to produce the race. However, the new trace output was also able to verify the fix when it was triggered during one of the test runs:

=== RUN   TestAppRole_TidyDanglingAccessors_RaceTest
2018-07-23T22:19:29.286-0400 [TRACE] tidy: listing role HMACs: prefix=secret_id/
2018-07-23T22:19:29.517-0400 [TRACE] tidy: listing accessors: prefix=accessor/
2018-07-23T22:19:30.150-0400 [TRACE] tidy: listing secret ID HMACs: role_hmac=96cf92d299266895ab08152a1c6b5e91f38fc2b0e59a0fbe03ce0c4b1e15b5a1/
2018-07-23T22:19:31.358-0400 [TRACE] tidy: found dangling accessor, verifying
2018-07-23T22:19:31.374-0400 [TRACE] tidy: found dangling accessor, verifying
--- PASS: TestAppRole_TidyDanglingAccessors_RaceTest (5.29s)
        path_tidy_user_id_test.go:143: wrote 31950 entries

The dangling accessor was triggered by the race (to which there isn't currently a straightforward fix, because of the complexity of locking the backend), but the validation added ensured that these valid accessors were rechecked and found to be correct, and did not remove them.

[vishalnayak]

This looks like a neat best effort to not remove valid accessor entries. LGTM!