lease: retry errors claiming lease in LeaseManager::spawn

[hawkw]

Currently, if the ensure_claimed call in the LeaseManager::spawn
background task fails, the entire task will bail and terminate
immediately. This is not desirable, as the intended use of
LeaseManager::spawn is to spawn a single background task that will
manage leasses for the entire lifetime of a controller process, and it
should not terminate in the face of a transient error claiming the
lease.

This branch changes LeaseManager::spawn to retry transient errors with
an exponential backoff strategy. Exponential backoffs are implemented
using the backoff crate, but --- because this dependency is not
exposed in the public API -- this is strictly an implementation detail,
and the dependency can be swapped out for another exponential backoff
implementation later, if desired. Errors marked as transient and retried
include network errors communicating with the API server, errors parsing
the API server response, and HTTP error statuses returned by the API
server. Errors that result from a bad configuration or missing
permissions are treated as fatal, as they are unlikely to ever succeed
after retrying.

Closes #137
Closes #136