chg: dev: Initialize the DST subsystem implicitly

Instead of calling dst_lib_init() and dst_lib_destroy() explicitly by
all the programs, create a separate memory context for the DST subsystem
and use the library constructor and destructor to initialize the DST
internals.

Merge branch 'ondrej/move-dst_lib_init-and-destroy-to-lib-ctor-dtor' into 'main'

See merge request isc-projects/bind9!9254