change math/rand to crypto/rand to break tie in elections

[CaojiamingAlan]

This is to solve #48
Change math/rand to crypto/rand so that even in emulations the random numbers are different for each server.
Result verified by the sample given in etcd-io/etcd#15784:

[ec2-user@ip-172-31-38-170 ~]$ cat shadow.data/hosts/server3/etcd.1000.stdout | grep leader
2000-01-01T00:00:02.65Z	info	raft	v3@v3.0.0-20221201111702-eaa6808e1f7a/node.go:344	raft.node: d88930ebb1eeba8b elected leader 4ec99d67dc5f5788 at term 2
[ec2-user@ip-172-31-38-170 ~]$ cat shadow.data/hosts/server1/etcd.1000.stdout | grep leader
2000-01-01T00:00:02.65Z	info	raft	v3@v3.0.0-20221201111702-eaa6808e1f7a/node.go:344	raft.node: 4f7b593e8bbade70 elected leader 4ec99d67dc5f5788 at term 2
[ec2-user@ip-172-31-38-170 ~]$ cat shadow.data/hosts/server2/etcd.1000.stdout | grep leader
2000-01-01T00:00:02.6Z	info	raft	v3@v3.0.0-20221201111702-eaa6808e1f7a/raft.go:903	4ec99d67dc5f5788 became leader at term 2
2000-01-01T00:00:02.6Z	info	raft	v3@v3.0.0-20221201111702-eaa6808e1f7a/node.go:344	raft.node: 4ec99d67dc5f5788 elected leader 4ec99d67dc5f5788 at term 2

[ahrtr]

LGTM

cc @tbg @pavelkalinnikov

[tbg]

(I verified that globalRand is used only for the election timeout, i.e. it's not on any hot path).