Fix #154 - relay dies with AssertionError

[jessedusty]

Fix race condition in the Java relay implementation due to timeout from no activity, it misses the cleaning deadline by a small number of milliseconds, and throws an exception because there are no open selectors.

Fixes #154

Evidence/log:
Used an additional print statement printing out the relevant variables and changed the original exception to a print

Selected keys size 1 now: 1562790544556 nextCleaningDeadline 1562790662669
Selected keys size 0 now: 1562790662667 nextCleaningDeadline 1562790662669
selector.select() returned without any event, an invalid SelectionKey was probably been registered
Selected keys size 0 now: 1562790662670 nextCleaningDeadline 1562790662669
Selected keys size 0 now: 1562790722672 nextCleaningDeadline 1562790722670

The print statement used was

System.out.println("Selected keys size " + selectedKeys.size() + " now: " + now + " nextCleaningDeadline " + nextCleaningDeadline);

Add this before line 56 of Genymoble:master Relay.java if you want to recreate it

Environment - java relay running on macOS 10.14

[rom1v]

Thank you for your PR.

I need to dive back into the code and think about it (but I have other things to do before, so it might take some time).

[rom1v]

Thank you very much, and sorry for the delay.

I finally found few hours tonight to work on gnirehtet. I rebased your commit onto current master and merged it as c814bfb.