* Sync up Agent and API's renewers.
This introduces a new type, LifetimeWatcher, which can handle both
renewable and non-renewable secrets, modeled after the version in Agent.
It allows the user to select behavior, with the new style being the
default when calling Start(), and old style if using the legacy Renew()
call.
No tests have been modified (except for reflect issues) and no other
code has been modified to make sure the changes are backwards
compatible.
Once this is accepted I'll pull the Agent version out.
* Move compat flags to NewRenewer
* Port agent to shared lifetime watcher lib
5c3649defe