ddclient: T5574: Support per-service cache management for services

Add support for per-service cache management for ddclient providers
via wait-time and expiry-time options. This allows for finer-grained
control over how often a service is updated and how long the hostname
will be cached before being marked expired in ddclient's cache.

More specifically, wait-time controls how often ddclient will attempt
to check for a change in the hostname's IP address, and expiry-time
controls how often ddclient to a forced update of the hostname's IP
address.

These options intentionally don't have any default values because they
are provider-specific. They get treated similar to the other provider-
specific options in that they are only used if defined.