rework hls segmenter (#407)

* rework hls segmenter to start more quickly

* don't use realtime encoding for hls until we're at least a minute ahead

* ugly but functional playlist filtering

ErsatzTV.Core/Errors/ChannelHasProcess.cs

@@ -1,9 +0,0 @@
-namespace ErsatzTV.Core.Errors
-{
-    public class ChannelHasProcess : BaseError
-    {
-        public ChannelHasProcess() : base("Channel already has ffmpeg process")
-        {
-        }
-    }
-}

ErsatzTV.Core/Errors/ChannelSessionAlreadyActive.cs

@@ -0,0 +1,9 @@
+namespace ErsatzTV.Core.Errors
+{
+    public class ChannelSessionAlreadyActive : BaseError
+    {
+        public ChannelSessionAlreadyActive() : base("Channel already has HLS session")
+        {
+        }
+    }
+}