[C++] Use RecordBatch::AddColumn to update RecordBatch

[Rares Vernica]

Hello,

I'm using RecordBatch:;AddColumn to update a RecordBatch. Something like
this:

std::shared_ptr<arrow::RecordBatch> rb;
...
rb = rb->AddColumn(...)

Since AddColumn creates a new RecordBatch, is the memory taken by rb before
assignment being freed as expected.

Thanks!
Rares