Fix invalid memory access on the first pending batch receive callback

[BewareMyPower]

Motivation

pulsar-client-cpp/lib/ConsumerImplBase.cc

Lines 109 to 112 in 5940cb5

	OpBatchReceive& batchReceive = batchPendingReceives_.front();
	batchPendingReceives_.pop();
	lock.unlock();
	notifyBatchPendingReceivedCallback(batchReceive.batchReceiveCallback_);

In the code above, batchReceive is a reference to the first element in the queue (batchPendingReceives_). After pop(), the memory would be invalid to access, which might cause unexpected crash.

Modifications

Add a popBatchReceiveCallback method that moves the callback from the first element in batchPendingReceives_. Use this method to replace error-prone front() - pop() calls.

[BewareMyPower]

This is an example crash frame in production:

frame #3: 0x0000ffff6efa846c pulsar.node`pulsar::ConsumerImplBase::notifyBatchPendingReceivedCallback(this=0x0000ffff54009a60) at ConsumerImplBase.cc:112:43

[lhotari]

LGTM. Good work @BewareMyPower