[YSQL] Support txn batch size for COPY to improve performance

[ndeodhar]

Today for improving COPY performance, we have a non-txn-copy gflag that can be used to run copy in non transactional mode. However, this does not take effect if table requires transactions (for example, if table has an index).

We should add a copy_txn_batch_size option to improve COPY performance for such transactional tables. This will reduce the batch size for transactions instead of loading all the data in a single large transaction.

[mbautin]

It might be useful to expose these COPY behavior options as GUCs, so that we could say e.g. SET yb_copy_txn_batch_size=100 in a session.