Live: Limit cached max frames by gop_cache_max_frames

[bluestn]

Use gop_cache_max_frames to limit the max frames which can be cached by gop cache, to keep srs from run out of memory while some malicious stream pusher never send any Key frames.

[bluestn]

OK, modified as you pointed out. thank u.

How to verify this patch can work?

1. find a flv video which is long enough encoded with avc/aac.
2. use the flv key frame remove tool (https://github.com/bluestn/flv_remove_keyframe) to remove the flv key frames.

git clone https://github.com/bluestn/flv_remove_keyframe.git
mkdir -p flv_remove_keyframe/build
cd flv_remove_keyframe/build
cmake ..
make
cp ~/git/srs/trunk/doc/source.flv .
./flv_remove_keyframe source.flv

3. use command to push the flv file to srs server.

ffmpeg -re -i source_no_keyframe.flv -c copy -f flv rtmp://localhost/live/livestream 

4. without this patch, and when gop_cache is on, we can easily see that srs server will cache more and more frames in the gop_cache, and finally oom sooner or later.
   and after this patch applied, gop_cache will be kept no more than the frames set by gop_cache_max_frames.

[winlinvip]

Verified, it works! 👍

Before this patch, the gop cache increases:
dispatch cached gop success. count=4244, duration=62760

After this patch, we will clear the gop cache:
Gop cache exceed max frames=250, total=251, videos=93, aalvc=1

And the memory is ok now.