Enable the implicit "max-cache-size 90%;" default to be overridden

!3865 (merged) caused RBT hash tables to be pre-allocated. This makes named use more memory immediately from startup; how much more memory it uses depends on the size of memory available on the host as the default value of max-cache-size is 90% (and the hash table size is derived from that value).

This is not expected to cause problems on systems which run a single instance of BIND, but it may trigger memory use issues e.g. on CI hosts, which run numerous instances of BIND in parallel and each of these instances assumes it is okay for it to use all of the memory available on the host. The most prominent display of that problem was addressed by !3919 (merged), but certain issues are still manifesting themselves intermittently - most notably, the FreeBSD QEMU VMs are often being killed off, which causes the FreeBSD GitLab CI jobs to appear to be hung, e.g.:

Since named instances used in BIND system tests do not really need large caches, what would address these memory use issues is a way of overriding the default max-cache-size 90%; setting present in bin/named/config.c. Any mechanism implemented for that purpose would still need to honor explicit max-cache-size settings present in named.conf.

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