Fixing synchronization issues in TcpConnection and CircularQueue

mSocket in EtchTcpConnection and mItems in CircularQueue can be accessed
and modified from different threads. Till now these members have not been
protected during shutdown (close() and destructor).

Change-Id: Iaec4a6a43c84c87ad897dea2be355d35903a98ec

git-svn-id: https://svn.apache.org/repos/asf/etch/trunk@1580795 13f79535-47bb-0310-9956-ffa450edef68