storage: Goroutine leaks in raft transport

[bdarnell]

The additional client streams created for snapshot messages in the raft transport are never closed unless the transport becomes completely idle. In #5970, we saw 19440 server-side goroutines in node 2, and the corresponding client-side goroutines on node 0 (15499) and node 3 (3961).

node0.stacks.txt
node1.stacks.txt
node2.stacks.txt
node3.stacks.txt
node4.stacks.txt