tokencache: respect last-modified of s3 cache before updating

[tripodsan]

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.