Update connection pinning logic for LB mode

The rules for connection pinning in LB mode have changed. For cursors:

1. No pinning if the cursor ID is 0 (i.e. all results are returned in a single batch)
2. Unpin once the cursor ID returned from getMore is 0 (i.e. the cursor has been drained)
3. Unpin once the cursor is closed.

For transactions, the rules will be the same as those for pinning sessions to servers in non-LB sharded transactions.