Defines mem-alloc function optionally weak, so users can override them
and implement their own allocation, or a static/bss memory for the mdns
task stack.
Original issue (data race when updating hostname): mdns_service_add()
makes a copy of the local hostname and calls using the local copy
mdns_service_add_for_host().
When mdns's hostname is updated the local copy gets out of sync.
**API race issue**: Most of the current API correctly lock the mdns service,
but sometimes unlocks it before sending an action to the action queue,
so it's possible that the situation changes before the actual action
takes place.
**Fix**: After locking the mdns service, we proceed directly with updating
internal structures and do not post actions into the action queue.
**Fix wrtt hostname**: Use mdns_service_add_for_host(hostname=NULL)
for all self hosted services.
MAJOR CHANGE: Fixed mdns API issues when add/remove/update records from multiple threads
This and the following commits fix the API race issues for these mdns APIs:
* mdns_service_add_for_host
* mdns_service_port_set_for_host
* mdns_service_txt_set_for_host
* mdns_service_txt_item_set_for_host_with_explicit_value_len
* mdns_service_txt_item_remove_for_host
* mdns_service_subtype_add_for_host
* mdns_service_instance_name_set_for_host
* mdns_service_remove_for_host
* mdns_service_remove_all
