Our handling for concurrent refresh of access tokens suffered from
a race condition where:

1. Thread A has just finished with updating the existing token
document, but hasn't stored the new tokens in a new document
yet
2. Thread B attempts to refresh the same token and since the
original token document is marked as refreshed, it decrypts and
gets the new access token and refresh token and returns that to
the caller of the API.
3. The caller attempts to use the newly refreshed access token
immediately and gets an authentication error since thread A still
hasn't finished writing the document.

This commit changes the behavior so that Thread B, would first try
to do a Get request for the token document where it expects that
the access token it decrypted is stored(with exponential backoff )
and will not respond until it can verify that it reads it in the
tokens index. That ensures that we only ever return tokens in a
response if they are already valid and can be used immediately

It also adjusts TokenAuthIntegTests
to test authenticating with the tokens each thread receives,
which would fail without the fix.

Resolves: elastic#54289