chg: dev: Add and use global memory context called isc_g_mctx

Instead of having individual memory contexts scattered across different
files and called different names, add a single memory context called
isc_g_mctx that replaces named_g_mctx and various other global memory
contexts in various utilities and tests.

Merge branch 'ondrej/add-global-isc_g_mctx-instance' into 'main'

See merge request isc-projects/bind9!10737