Add per channel storage durations

This lets us save images for less time than everything else, since they
are the main driver of queue memory usage.

Change-Id: I677791e20af6aedb0288d09cf5963b4dde30ecce
Signed-off-by: James Kuszmaul <james.kuszmaul@bluerivertech.com>
diff --git a/aos/configuration.fbs b/aos/configuration.fbs
index 5283d9e..0b42f5b 100644
--- a/aos/configuration.fbs
+++ b/aos/configuration.fbs
@@ -105,6 +105,10 @@
   //
   // Currently, this must be set if and only if read_method is PIN.
   num_readers:int (id: 12);
+
+  // Length of this channel in nanoseconds.  This overrides
+  // channel_storage_duration below in Configuration for just this channel.
+  channel_storage_duration:long = 2000000000 (id: 13);
 }
 
 // Table to support renaming channel names.