Add delegdb configuration struct

Add delegdb configuration struct

Instead of having independent APIs to configure various aspects of the
delegdb (i.e. cache size, other settings that may come up later), a
single configuration struct is passed to `dns_delegdb_setconfig()`, which
internally does all the plumbing. To avoid relying on
atomics/synchronization, `dns_delegdb_setconfig()` must be called from
exclusive mode (for now).

The configuration can be retrieved at any time (not necessarily from
exclusive mode) using `dns_delegdb_getconfig()`. This is useful, for
instance, to flush the delegdb without losing its parameters.