serf: protect field access with lock to avoid potential data race

[yanke-xu]

I found a potential data race based on the code analysis. For example, there are two confilicting operations on the field "EventLTime", where the read operation at: serf/delegate.go:223, and the write operation which is protected by the eventLock at: serf/delegate.go:264. Similar to preventing write conflicts on line 264, the two operations may cause a data race.