SPY-192: Ensure connection key is valid before checking if writable.

Motivation
----------
It has been discovered in the wild that sometimes CancelledKeyException are
received even if the key has been checked for validity before. It turns out
that the validity can change between the check and the next function call.

Modifications
-------------
The changeset makes sure to check if it is valid on both reading and
subsequent writing, reducing the chance of race conditions and interleaved
state changes.

Result
------
More stable behavior at runtime and less chance of CancelledKeyException.

Change-Id: Ic0db65f9c7ccc3a7b9738aaff0e3c7e60d7f25c2
Reviewed-on: http://review.couchbase.org/72270
Reviewed-by: Michael Nitschinger <michael@nitschinger.at>
Tested-by: Michael Nitschinger <michael@nitschinger.at>

Author: ggrochow

src/main/java/net/spy/memcached/MemcachedConnection.java

@@ -713,18 +713,15 @@ private void handleIO(final SelectionKey sk) {
    * @param node th enode to read write from.
    * @throws IOException if an error occurs during read/write.
    */
-  private void handleReadsAndWrites(final SelectionKey sk,
-    final MemcachedNode node) throws IOException {
-    if (sk.isValid()) {
-      if (sk.isReadable()) {
-        handleReads(node);
+  private void handleReadsAndWrites(final SelectionKey sk, final MemcachedNode node) throws IOException {
+      if (sk.isValid() && sk.isReadable()) {
+          handleReads(node);
       }
-      if (sk.isWritable()) {
-        handleWrites(node);
+
+      if (sk.isValid() && sk.isWritable()) {
+          handleWrites(node);
       }
-    }
   }
-
   /**
    * Finish the connect phase and potentially verify its liveness.
    *