Migrate from NGINX-RTMP, support exec_on_dvr

[winlinvip]

#367 (comment)

NGINX-RTMP:
https://github.com/arut/nginx-rtmp-module/wiki/Directives#exec_record_done
Command executed upon completion of the DVR.

recorder - recorder name
path - recorded file path (/tmp/rec/mystream-1389499351.flv)
filename - path with directory omitted (mystream-1389499351.flv)
basename - file name with extension omitted (mystream-1389499351)
dirname - directory path (/tmp/rec)

https://github.com/arut/nginx-rtmp-module/wiki/Directives#on_record_done
NGINX-RTMP also provides HTTP callbacks, similar to SRS's on_dvr.

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[winlinvip]

SRS supports the command "exec_on_dvr" when using DVR, which is similar to "exec_record_done" in NGINX-RTMP.

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[winlinvip]

I haven't decided whether to support exec yet, so let's put this feature on hold.

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[winlinvip]

#459 (comment)
DVR API has been supported already.

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[winlinvip]

Also supported EXEC, https://github.com/simple-rtmp-server/srs/wiki/v3_CN_NgExec.
But not planning to support exec on dvr.

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