[server] Suppress an uninitialized value is used warning

It is reported on CentOS 6.5 like the following:

    ../src/AtomicValue.h: In function 'void testAtomicValue::test_setAndGet()':
    ../src/AtomicValue.h:44: warning: 'val' is used uninitialized in this function
    testAtomicValue.cc:49: note: 'val' was declared here

By the way, should we support AtomicValue constructor without initial
value?