Make EventCounter thread safe

[DomGarguilo]

Fixes #4956

The stack track included in the ticket indicates that a ConcurrentmodificationException occurred in Monitor.EventCounter:

java.util.ConcurrentmodificationException
at java.util.HashMap$HashIterator.nextNode(HashMap.java:1511)
at java.util.HashMap$HashIterator.next(HashMap.java:1544)
at java.util.HashMap$HashIterator.next(HashMap.java:1542)
at org.apache.accumulo.monitor.Monitor$EventCounter.calculateRate(monitor.java:23)
at org.apache.accumulo.monitor.Monitor.getLookupRate(Monitor.java:977)
at org.apache.accumulo.monitor.rest.tservers.TabletServerInformation.updateTabletServerInfo(TabletServerinformation.java:162)
at org.apache.accumulo.monitor.rest.tservers.TabletServerInformation.(TabletServerInformation:101)
at org.apache.accumulo.monitor.rest.tservers.TabletServer.(TabletServer.java:43)
at org.apache.accumulo.monitor.rest.tservers.TabletServerResource.getTserverSummary(TabletServerResource.java:88)

This PR adds synchronization to several methods in the EventCounter class to avoid this from happening again.

Another approach that would probably work would be to make the 3 collections in this class thread safe by making the HashMaps into ConcurrentHashMaps and the HashSet into a Collections.synchronizedSet(new HashSet<>()). That would function just fine in the current code as far as I can tell but if we would ever need to iterate over the synchronizedSet, we would need to use a synchronize block anyways so i figured just making the methods themselves synchronized would future proof and fix the current issue too.