Merge branch 'kn/osxkeychain-skip-idempotent-store'

The credential helper that talks with osx keychain learned to avoid
storing back the authentication material it just got received from
the keychain.

* kn/osxkeychain-skip-idempotent-store:
  osxkeychain: state to skip unnecessary store operations
  osxkeychain: exclusive lock to serialize execution of operations