tokencache: respect last-modified of s3 cache before updating

Current situation:
- at time T1, instance A loads the S3 token
- at time T2, instance B loads the S3 token
- instance B refreshes the token, stores it back to S3
- later, instance A also refreshes the token, and **writes it back as well**, thereby overwriting a perfectly valid token stored by instance B

Instead, instance A should check that the `Last-Modified` value of the stored token is still the one it has seen when loading in T1. At least, it should internally store the new `Last-Modified` value, to compare it again when the next refresh happens.