ChannelOutboundBuffer bytes before writable accessor

Motiviation:
There are currently no accessors which provide visbility into how many bytes must be written in order for a writability change to occur. This feature would be useful for codecs which intent to control how many bytes are queued at any given time.

Modifications:
- add bytesBeforeUnWritable() which will give the number of bytes before the buffer (and associated channel) transitions to not writable
- add bytesBeforeWritable() which will give the number of bytes that must be drained from the queue until the channel becomes writable.

Result:
More visibility into writability for the ChannelOutboundBuffer.

transport/src/main/java/io/netty/channel/ChannelOutboundBuffer.java

@@ -689,6 +689,36 @@ public final class ChannelOutboundBuffer {
         return totalPendingSize;
     }
 
+    /**
+     * Get how many bytes can be written until {@link #isWritable()} returns {@code false}.
+     * This quantity will always be non-negative. If {@link #isWritable()} is {@code false} then 0.
+     */
+    public long bytesBeforeUnWritable() {
+        long bytes = channel.config().getWriteBufferHighWaterMark() - totalPendingSize;
+        // If bytes is negative we know we are not writable, but if bytes is non-negative we have to check writability.
+        // Note that totalPendingSize and isWritable() use different volatile variables that are not synchronized
+        // together. totalPendingSize will be updated before isWritable().
+        if (bytes > 0) {
+            return isWritable() ? bytes : 0;
+        }
+        return 0;
+    }
+
+    /**
+     * Get how many bytes must be drained from the underlying buffer until {@link #isWritable()} returns {@code true}.
+     * This quantity will always be non-negative. If {@link #isWritable()} is {@code true} then 0.
+     */
+    public long bytesBeforeWritable() {
+        long bytes = totalPendingSize - channel.config().getWriteBufferLowWaterMark();
+        // If bytes is negative we know we are writable, but if bytes is non-negative we have to check writability.
+        // Note that totalPendingSize and isWritable() use different volatile variables that are not synchronized
+        // together. totalPendingSize will be updated before isWritable().
+        if (bytes > 0) {
+            return isWritable() ? 0 : bytes;
+        }
+        return 0;
+    }
+
     /**
      * Call {@link MessageProcessor#processMessage(Object)} for each flushed message
      * in this {@link ChannelOutboundBuffer} until {@link MessageProcessor#processMessage(Object)}