i2psnark: Don't count unchoked but uninterested peers as uploaders

when calculating global uploader limit, to allow more upload slots,
especially when some torrents are stalled.
Convert some shared fields to atomics for concurrency.

apps/i2psnark/java/src/org/klomp/snark/Snark.java

@@ -1337,7 +1337,7 @@ public class Snark
     int totalUploaders = 0;
     for (PeerCoordinator c : _peerCoordinatorSet) {
       if (!c.halted())
-        totalUploaders += c.uploaders;
+        totalUploaders += c.getInterestedUploaders();
     }
     int limit = _util.getMaxUploaders();
     if (_log.shouldLog(Log.DEBUG))