process_pending_handles: mark queued transfers as previously pending

With transfers being queued up, we only move one at a a time back to the
CONNECT state but now we mark moved transfers so that when a moved
transfer is confirmed "successful" (it connected) it will trigger the
move of another pending transfer. Previously, it would otherwise wait
until the transfer was done before doing this. This makes queued up
pending transfers get processed (much) faster.