Extend file analysis API to allow file ID caching, adapt HTTP to use it.

This allows an analyzer to either provide file IDs associated with some
file content or to cache a file ID that was already determined by
script-layer logic so that subsequent calls to the file analysis
interface can bypass costly detours through script-layer.  This can
yield a decent performance improvement for analyzers that are able to
take advantage of it and deal with streaming content (like HTTP).

src/analyzer/protocol/http/HTTP.h

@@ -64,6 +64,7 @@ protected:
 	uint64_t offset;
 	int64_t instance_length; // total length indicated by content-range
 	bool send_size; // whether to send size indication to FAF
+	std::string cached_file_id;
 
 	MIME_Entity* NewChildEntity() { return new HTTP_Entity(http_message, this, 1); }