prevent int64 overflow for default_lease_ttl and max_lease_ttl

[ccapurso]

It is possible for vault read sys/config/state/sanitized to result in int64 overflow for large values of default_lease_ttl and max_lease_ttl. This occurs in the duration parsing in humanDurationInt in base_helpers.go. In order to address this, the unit used has been changed from nanoseconds to seconds in core.Config.Sanitzed(). While this fixes the integer overflow issue, it also changes the contract of the API which is why the changelog lists a change instead of bug.

Fixes #14175.