JWT: require valid `exp` claim by default

Users of JWT rightly expect tokens to be considered invalid once they expire. It
is a surprise to some that this requires a change to the default
configuration. In the interest of security we will now require a valid `exp`
claim in tokens. Administrators can disable the check by changing
`required_claims` back to the empty string.

We do not add `nbf` as a required claim as it seems to not be set often in
practice.

closes https://github.com/apache/couchdb/issues/5046
2 files changed