Ignore jemalloc versions before 4.0.0

We now depend on explicitly creating memory arenas and disabling tcache on those, and these features are not available with jemalloc < 4. Instead of working around these issues, make the jemalloc >= 4.0.0 hard requirement by looking for sdallocx() symbol that's only available from that version.

The jemalloc < 4 was only used by RHEL 7 which is not supported since BIND 9.19+.

Closes #4296 (closed)