Log every exception thrown from Memcached IO thread

Related to BES-16032, this patch helps to gather more information about
the issue encountered that kill the Memcached IO thread.

Please note that this patch will not kill the Memcached IO thread which
will continue to run.

Previous known bug reports related to this issue are:
couchbase#7
couchbase#17

Change-Id: If1d9887963e96ac6ee4a8ec0dd0c19626e521346
JIRA:WBSC-3570

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

@@ -1467,6 +1467,8 @@ public void run() {
         logRunException(e);
       } catch (ConcurrentModificationException e) {
         logRunException(e);
+      } catch (Exception e) {
+        getLogger().error("Unexpected exception catch by Memcached IO thread ", e);
       }
     }
     getLogger().info("Shut down memcached client");