text: Title: empty ${DESECSTACK_API_PSL_RESOLVER} breaks POSTing domains
  Body: Setting `${DESECSTACK_API_PSL_RESOLVER}` to empty (or not setting it at all) in `.env` will result in a 30s delay when posting to `api/v1/domains` endpoint, then raise a timeout exception, which results in a 500 error.
  
  Call stack:
  
  	api_1       | Internal Server Error: /api/v1/domains/
  	api_1       | Traceback (most recent call last):
  	api_1       |   File "/usr/local/lib/python3.7/site-packages/django/core/handlers/exception.py", line 34, in inner
  	api_1       |     response = get_response(request)
  	api_1       |   File "/usr/local/lib/python3.7/site-packages/django/core/handlers/base.py", line 115, in _get_response
  	api_1       |     response = self.process_exception_by_middleware(e, request)
  	api_1       |   File "/usr/local/lib/python3.7/site-packages/django/core/handlers/base.py", line 113, in _get_response
  	api_1       |     response = wrapped_callback(request, *callback_args, **callback_kwargs)
  	api_1       |   File "/usr/local/lib/python3.7/site-packages/django/views/decorators/csrf.py", line 54, in wrapped_view
  	api_1       |     return view_func(*args, **kwargs)
  	api_1       |   File "/usr/local/lib/python3.7/site-packages/django/views/generic/base.py", line 71, in view
  	api_1       |     return self.dispatch(request, *args, **kwargs)
  	api_1       |   File "/usr/local/lib/python3.7/site-packages/rest_framework/views.py", line 495, in dispatch
  	api_1       |     response = self.handle_exception(exc)
  	api_1       |   File "/usr/local/lib/python3.7/site-packages/rest_framework/views.py", line 455, in handle_exception
  	api_1       |     self.raise_uncaught_exception(exc)
  	api_1       |   File "/usr/local/lib/python3.7/site-packages/rest_framework/views.py", line 492, in dispatch
  	api_1       |     response = handler(request, *args, **kwargs)
  	api_1       |   File "/usr/local/lib/python3.7/site-packages/rest_framework/generics.py", line 244, in post
  	api_1       |     return self.create(request, *args, **kwargs)
  	api_1       |   File "/usr/local/lib/python3.7/site-packages/rest_framework/mixins.py", line 21, in create
  	api_1       |     self.perform_create(serializer)
  	api_1       |   File "./desecapi/views.py", line 119, in perform_create
  	api_1       |     public_suffix = self.psl.get_public_suffix(domain_name)
  	api_1       |   File "/usr/local/lib/python3.7/site-packages/psl_dns/querier.py", line 42, in get_public_suffix
  	api_1       |     public_suffix = self._get_public_suffix_raw(domain)
  	api_1       |   File "/usr/local/lib/python3.7/site-packages/psl_dns/querier.py", line 30, in _get_public_suffix_raw
  	api_1       |     answer = self.query(domain, dns.rdatatype.PTR)
  	api_1       |   File "/usr/local/lib/python3.7/site-packages/psl_dns/querier.py", line 93, in query
  	api_1       |     answer = self.resolver.query(qname, rdatatype, lifetime=self.timeout)
  	api_1       |   File "/usr/local/lib/python3.7/site-packages/dns/resolver.py", line 992, in query
  	api_1       |     timeout = self._compute_timeout(start, lifetime)
  	api_1       |   File "/usr/local/lib/python3.7/site-packages/dns/resolver.py", line 799, in _compute_timeout
  	api_1       |     raise Timeout(timeout=duration)
  	api_1       | dns.exception.Timeout: The DNS operation timed out after 30.001466035842896 seconds
  	api_1       | [pid: 250|app: 0|req: 1/1] 172.16.0.1 () {44 vars in 629 bytes} [Thu May 30 17:31:09 2019] POST /api/v1/domains/ => generated 14294 bytes in 30219 msecs (HTTP/1.1 500) 2 headers in 102 bytes (1 switches on core 0)
  
  Expected behavior: according to README: use the system's resolver. (I confirmed in my setup that the resolver is working; however wireshark did not show a DNS query to somewhere after trying to post a domain.)
  
  Steps to reproduce: clean master, clean builds, empty database, unset psl resolver (obviously). Then post to the domains endpoint.
  
  Workaround: set it to 9.9.9.9 or competitors.