When `Range` was called on an empty ttlcache object, the older version
of ttlcache would trigger a process panic.

We will need version information initialized in the `server_structs`
module; having `server_structs` include the `config` module for the
version information causes a circular dependency.

This factors out the version-related calls (keeping backward API
compatibility) into a new module, breaking the dependency.

With this, an empty configuration string indicates to use the default.

The common base includes the name -- but also the version number,
a process/instance identifier, and a generation number.

The instance ID and generation number provide a total ordering for
all ads generated by the name/type of ad.  This will allow a process
to compare any two ads and determine which one is the "newer" of the
two.  This will allow the director to keep the right ad in memory
even if it receives them out-of-order.

The director ad needs to be able to determine its own name, just like
the cache and origin.  Pull these into a single location.

There's a need for future improvements -- the service should persist
its currently registered name -- but this simple refactor should suffice
for this line of work.

This provides a helper function to ensure we consistently calculate
the audience value for our token configuration.

Provide a framework for any service to detect the presence of multiple
director services in a federation, whether from static configuration,
from metadata discovery, or from querying known directors.  All the
sources are merged together to create one set of known directors.

Once the directors are known, and advertisements from the service
are sent to all directors directly from the service.

Additionally, the directors will forward the ads they receive from
a service to all other known directors.  This prevents a director
from missing updates just because the service hadn't discovered it
yet.

The directors will compare the generation and instance IDs of the
incoming ads to remove duplicates and ensure they only keep the
latest copy of a given service description.

A simple unit test is included.