[ISC-support #17264] add configuration option to set ADB size independent of cache size

Currently the maximum ADB size is set as a fraction (1/8) of the configured/effective max-cache-size.

It has been observed with at least one customer that this size is sometimes insufficient.

This may be related to the rise of CDNs with long CNAME chains spread across multiple domains with each domain typically hosted by a large group of geographically-diverse authoritative servers.

While there are other features necessary before an operator can tune this based on data rather than intuition, it may also be useful in our own explorations of ADB behavior in adverse conditions.