Fix large download stuck with H2 priority

[shinrich]

Was trying enabling priorities with HTTP/2 again. Fetching a 1.2MB file from cache would frequently (almost always) fail. It would send the first part and then stall. Eventually things would timeout and abort.

Looking at the debug messages, we saw that many data frames were sent. Then we would hit "No payload". Nothing more would happen on that thread until 30 seconds later when the state machine would cleanup because of inactivity timeout.

Talking with @SolidWallOfCode it appears that we need to signal WRITE_READY to the cache vc after we drain the buffer, so it will write more data into the buffer. This is done in the non-priority case.

I saw all this on our internal branch, so I am not 100% sure this is an issue in open source. But I'm heading out for a week, so I wanted to push this issue up before I left in case it is a general issue on open source master.

I saw some other irregularities with the priority processing which I don't have a fix for. Will file an issue for that.

[shinrich]

[approve ci autest]

[shinrich]

[approve ci clang-analyzer]

[maskit]

I cannot reproduce the issue on master. Can you confirm this is an issue in public version too?

Also, there is a similar function, Http2Stream::signal_write_event. I'm not sure if this can be an alternative, but calling it in Http2ConnectionState::send_data_frames_depends_on_priority would look more reasonable than adding a similar function and call it in send_a_data_frame.

[shinrich]

Indeed, it looks lovely on master.