Fix thread synchronization issues about greenmail

[buildscientist]

I am unable to reproduce this - did you run the test multiple times in separate JVM processes? Maybe I can use JMeter to generate enough load to force this issue to happen.

[camann9]

No, just run test20Senders20x4RetrieversAtTheSameTime from your IDE. Maybe increase the number of threads by increasing num.
I just tried with num=50 and found an exception:

5010    WARN           imap.ImapHandler| Can not close output stream
java.lang.NullPointerException
at com.icegreen.greenmail.imap.ImapHandler.close(ImapHandler.java:142)
at com.icegreen.greenmail.imap.ImapHandler.run(ImapHandler.java:100)
at com.icegreen.greenmail.server.AbstractServer$1.run(AbstractServer.java:101)
at java.lang.Thread.run(Thread.java:744)

Probably there are more of them, though I already fixed some.

[marcelmay]

Sonar reports 2 blocker issues "Methods "wait(...)", "notify()" and "notifyAll()" should never be called on Thread instances" for AbstractServer and Service. These issues should also be included.