[core] Set disk cache size to 0 by default

I'm pretty confident that we can safely keep MemoryWriter available
for people who wish to slightly improve performance and try out
this experimental feature.

Default it to 0 in size so it's effectively off. Someone can set
this to a reasonable value (5MB?) if they wish to help test it
in the real world.

src/MonoTorrent/MonoTorrent.Client/Settings/EngineSettings.cs

@@ -74,9 +74,9 @@ public class EngineSettings : IEquatable<EngineSettings>
 
         /// <summary>
         /// Creates a cache which buffers data before it's written to the disk, or after it's been read from disk.
-        /// Defaults to 5MB (5 * 1024 * 1024).
+        /// Experimental feature. Currently defaults 0. Recommended value when experimenting is 5MB (5 * 1024 * 1024).
         /// </summary>
-        public int DiskCacheBytes { get; } = 5 * 1024 * 1024;
+        public int DiskCacheBytes { get; } = 0;
 
         /// <summary>
         /// The UDP port used for DHT communications. Use 0 to choose a random available port.