Correct the returning type of Connection::waiting()
Correct the returning type of Connection::waiting()

The returning type of Connection::waiting() should be bool,
but currently it's size_t type.

CopernicaMarketingSoftware/AMQP-CPP,toolking/AMQP-CPP,CopernicaMarketingSoftware/AMQP-CPP,fantastory/AMQP-CPP,toolking/AMQP-CPP,fantastory/AMQP-CPP