Replace BlockingQueue with a concurrent version so that readers and writers don't block

Data Of Interest (CPU Metrics)
2 of 14: 30.6% (1.309s of 4.279s)

libpthread.so.0!pthread_cond_signal - [Unknown]
impalad!impala::ExecNode::RowBatchQueue::GetBatch+0x158 - [Unknown]:[Unknown]
impalad!impala::HdfsScanNode::GetNextInternal+0xf0 - [Unknown]:[Unknown]
impalad!impala::HdfsScanNode::GetNext+0x2fc - [Unknown]:[Unknown]
impalad!impala::PartitionedAggregationNode::Open+0x344 - [Unknown]:[Unknown]
impalad!impala::PlanFragmentExecutor::OpenInternal+0x92 - [Unknown]:[Unknown]
impalad!impala::PlanFragmentExecutor::Open+0x2d8 - [Unknown]:[Unknown]
impalad!impala::FragmentMgr::FragmentExecState::Exec+0x1f - [Unknown]:[Unknown]
impalad!impala::FragmentMgr::FragmentExecThread+0x4b - [Unknown]:[Unknown]
impalad!impala::Thread::SuperviseThread+0x1b9 - [Unknown]:[Unknown]
impalad!boost::detail::thread_data<boost::_bi::bind_t<void, void (*)(std::string const&, std::string const&, boost::function<void (void)>, impala::Promise<long>*), boost::_bi::list4<boost::_bi::value<std::string>, boost::_bi::value<std::string>, boost::_bi::value<boost::function<void (void)>>, boost::_bi::value<impala::Promise<long>*>>>>::run+0x7f - [Unknown]:[Unknown]
impalad!func@0xd18ce0+0x62 - [Unknown]:[Unknown]
libpthread.so.0!start_thread+0xd0 - [Unknown]:[Unknown]
libc.so.6!clone+0x6c - [Unknown]:[Unknown]

RowBatch* ExecNode::RowBatchQueue::GetBatch() {
  RowBatch* result = NULL;
  if (BlockingGet(&result)) return result;
  return NULL;
}