Max duration support for stream dumpfiles

We've received a request on this topic:

My suggestion is that the dump-file tag have an interval option or tag so 
that it creates a new dump file based on this interval, and named based 
on some sort of dump-file-name tag which would use BASH naming variables 
to name it.

http://lists.xiph.org/pipermail/icecast/2015-March/013209.html

Basically boils down to setting a duration and after that reopening the dump file. We already support strftime patterns in the file name.

I have received a patch for this against 2.3.2 and we'll evaluate if it can be reused or at least used as inspiration.

Optional related feature: dump files triggered / turned on/off through admin request.