Slaves should handshake all their chained members at once

Currently when a secondary connects to its sync source for upstream position notification, it sends the replSetUpdatePosition command with the "handshake" argument once for itself and once for each member chained off it.

After the initial connection, the secondary sends replication progress upstream with a replSetUpdatePosition command that includes information about itself and all chained nodes.

We should make handshakes work the same way to avoid multiple network round trips. This may become even more valuable if we raise the max replica set size.