Add expect bootstrap '-expect=n' mode.

This allows for us to automatically bootstrap a cluster of nodes after
'n' number of server nodes join. All servers must have the same 'n' set, or
they will fail to join the cluster; all servers will not join the peer set
until they hit 'n' server nodes.

If the raft commit index is not empty, '-expect=n' does nothing because it
thinks you've already bootstrapped.

[comments/criticisms/style tips/life tips appreciated]
Should this include s2's addr as well?
(tests are suddenly failing because s1 and s2 join (which is okay, so they both see 2 nodes), then s1 and s3 join (and they see 3 nodes, and so they leave expect mode) but s2 does not see s3...