Tablet copy session may expire before completion for large tablet

KUDU-1726 introduced an optimization to do a bulk sync-to-disk once the tablet copy operation is complete. However, when tested in a large cluster, I found disk synchronization in batches can result in the tablet session expires before the synchronization complete. There is a flag '--tablet_copy_idle_timeout_ms' to control the amount of time without activity before a tablet copy session expires, but it is tagged as hidden(not user-facing).