replace lua-socket depedency with lua-http

At the moment we are using two packages for HTTP requests from Lua:

• lua-socket (Lua library ssl.https)
• lua-http (Lua library http)

This complicates packaging and is generally unnecessary.

It seems that package lua-socket (Lua library ssl.https) offers only blocking API, and that is causing problems like e.g. #512 (closed), so let's replace lua-socket with lua-http.

It should "accidentally" fix #512 (closed) and also make packaging easier.

Affected modules:

• prefill (#512 (closed))
• trust_anchors bootstrap
• possibly others

Example of a non-blocking HTTP request:

function blacklist_reload()
	local url = 'https://raw.githubusercontent.com/CSNOG/MFCR-blacklist/master/blacklist.txt'
	local headers, stream = http_request.new_from_uri(uri):go()
	assert(headers, 'HTTP client library error')
	assert(tonumber(headers:get(':status')) == 200,
		string.format('HTTP status %s instead of expected 200\n', headers:get(':status')))
	local tmpfile = stream:get_body_as_file(5)
	assert(tmpfile, 'error while getting blacklist HTTP body in limit 5 seconds')
end
worker.bg_worker.cq:wrap(blacklist_reload)

Error handling needs more work etc.