Semantics of FileMetaDataBuilder::Finish() and saving memory by moving std::vector<RowGroup> metadata

[kostrzewa9ld]

I've noticed that in FileMetaDataBuilderImpl::Finish() there is a copy of row_groups_ field done by passing it via const& to metadata_->__set_row_groups(row_groups_). However, if it is assumed that after calling the Finish() method nothing should be touching the row_groups_ field anymore then maybe it could be moved instead?

[raulcd]

In my opinion this makes sense, do you want to open an issue and submit a PR?

[kostrzewa9ld]

Sure, I will give it a try in a few days.