adb->hmctx does not have a name associated with it

This is a minor issue introduced by !7228 (merged) and its backport (!7248 (merged)). See #3739 (closed) for context.

The new memory context added in those MRs, adb->hmctx, does not have a name associated with it.

Before (e.g. BIND 9.18.10)

$ curl -s http://localhost:8080/json | jq ".memory.contexts[].name"
"main"
"zonemgr-pool"
"zonemgr-pool"
"clientmgr"
"clientmgr"
"clientmgr"
"clientmgr"
"cache"
"cache_heap"
"ADB"
"cache"
"cache_heap"
"ADB"

After (e.g. BIND 9.18.11)

$ curl -s http://localhost:8080/json | jq ".memory.contexts[].name"
"main"
"zonemgr-pool"
"zonemgr-pool"
"clientmgr"
"clientmgr"
"clientmgr"
"clientmgr"
"cache"
"cache_heap"
"ADB"
null
"cache"
"cache_heap"
"ADB"
null

Could we please add an isc_mem_setname() call for this new memory context that would describe its purpose?