Issue 5182, 4688 - improve IsSubscriptionDowloadAllowedCallback and corresponding tests

- remove LazyWebRequest (issue 4688) and fix tests, the rest is for issue 5182
- make make FilterEngine::IsConnectionAllowedCallback asynchronous (it's also renamed)
  FilterEngine does not use detached thread for it anymore, now it's a responsibility of a user of libadblockplus.
- rename member of FilterEngine::CreationParameters isConnectionAllowedCallback to isSubscriptionDowloadAllowedCallback
- rename FilterEngineIsAllowedConnectionTest to FilterEngineIsSubscriptionDownloadAllowedTest and make it less hacky.

# it seems too much, let me know if you think it should be split into commits.

# based on https://codereview.adblockplus.org/29435645/

[sergei, 2017-05-10 17:04:40]

https://codereview.adblockplus.org/29435650/diff/29435651/test/FilterEngine.cpp
File test/FilterEngine.cpp (left):

https://codereview.adblockplus.org/29435650/diff/29435651/test/FilterEngine.c...
test/FilterEngine.cpp:178: class SyncStrings
since there are no threads there is no need for some serialized access to the
container, so that class is replaced by
typedef std::vector<std::pair<bool, std::string>> ConnectionTypes;

https://codereview.adblockplus.org/29435650/diff/29435651/test/FilterEngine.c...
test/FilterEngine.cpp:184: strings.emplace_back(!!value, value ? *value : "");
this functionality is in the callback body now.

https://codereview.adblockplus.org/29435650/diff/29435651/test/FilterEngine.cpp
File test/FilterEngine.cpp (right):

https://codereview.adblockplus.org/29435650/diff/29435651/test/FilterEngine.c...
test/FilterEngine.cpp:214: for (const auto& isSubscriptionDowloadAllowedCallback
: isSubscriptionDowloadAllowedCallbacks)
actually, there should be only one callback and one corresponding web request.

[hub, 2017-05-10 18:53:53]

LGTM