Parse TFRF for continuous SmoothStreaming segments refresh

[phhusson]

This fixes issues #855

This is needed in our use-case because we have a backend for which the access to the manifest requires a token that is valid only for one minute.

I have tested this with http:https://essdrmproducts-dev.insidesecure.com:9090/live/clear.isml/manifest
I added logs in HttpDataSource to log every access to manifest and confirmed it is now only accessed once, while it was recurrent before.

Points that are unclear to me:

• When testing on http:https://essdrmproducts-dev.insidesecure.com:9090/live/clear.isml/manifest I can see that Exoplayer tries to download chunks ahead of their time, so the server returns few 412 before the chunks are actually available. This doesn't happen prior to the patch. Since I don't change the chunk download algorithm I'm not sure why this happen.
• I'm using System.currentTimeMillis() to expire old chunks. I think this is the sane way to do, but i'm not totally convinced

[guidofranceschini]

Hi phhusson ,
I am not among the contributors, but I can probably explain why you observe a few 412.
Within a manifest you can get information only about chunks that are fully available.
Instead. from the boxes within a chunk you can determine the timestamp and hence the URL of the next chunk, but this "next chunk" is not necessarily available yet. You have probably updated the code in a way that it now knows about the "next chunk" before it is actually available.