flaky test TestSyncTriggerDeleteExpriedKeys

Seeing sporadic failures of this:
```
2014/09/22 22:34:29 etcdserver: set snapshot count to default 10000
2014/09/22 22:34:30 raft: leader changed from 0x0 to 0x1
2014/09/22 22:34:30 etcdserver: set snapshot count to default 10000
2014/09/22 22:34:30 etcdserver: set snapshot count to default 10000
2014/09/22 22:34:30 etcdserver: set snapshot count to default 10000
2014/09/22 22:34:30 raft: leader changed from 0x0 to 0x1
2014/09/22 22:34:30 raft: leader changed from 0x0 to 0x1
2014/09/22 22:34:30 raft: leader changed from 0x0 to 0x1
2014/09/22 22:34:30 etcdserver: set snapshot count to default 10000
2014/09/22 22:34:30 raft: leader changed from 0x0 to 0xbad0
2014/09/22 22:34:30 etcdserver: set snapshot count to default 10000
2014/09/22 22:34:30 raft: leader changed from 0x0 to 0xbad0
2014/09/22 22:34:30 etcdserver: set snapshot count to default 10000
2014/09/22 22:34:30 raft: leader changed from 0x0 to 0xbad0
2014/09/22 22:34:30 etcdserver: set snapshot count to default 10000
2014/09/22 22:34:30 raft: leader changed from 0x0 to 0xbad0
2014/09/22 22:34:30 etcdserver: set snapshot count to default 10000
2014/09/22 22:34:30 raft: leader changed from 0x0 to 0xbad0
2014/09/22 22:34:30 etcdserver: set snapshot count to default 10000
--- FAIL: TestSyncTriggerDeleteExpriedKeys (0.00 seconds)
	server_test.go:469: len(action) = 0, want 1
2014/09/22 22:34:30 raft: leader changed from 0x0 to 0xbad0
2014/09/22 22:34:30 etcdserver: set snapshot count to default 10000
2014/09/22 22:34:30 etcdserver: set snapshot count to default 10000
FAIL
coverage: 91.7% of statements
FAIL	github.com/coreos/etcd/etcdserver	0.505s
```
What I mean to say is: either we decide that sync() *must* only be called as leader; or we decide that it's not so important when it's called.