changed sent and received to use ConcurrentHashMap, made sure non-null keys are added
changed sent and received to use ConcurrentHashMap, made sure non-null keys are added

pruivo/JGroups,vjuranek/JGroups,Sanne/JGroups,kedzie/JGroups,TarantulaTechnology/JGroups,ligzy/JGroups,pferraro/JGroups,rhusar/JGroups,kedzie/JGroups,Sanne/JGroups,TarantulaTechnology/JGroups,rpelisse/JGroups,tristantarrant/JGroups,rhusar/JGroups,danberindei/JGroups,dimbleby/JGroups,ibrahimshbat/JGroups,deepnarsay/JGroups,ligzy/JGroups,slaskawi/JGroups,ibrahimshbat/JGroups,dimbleby/JGroups,deepnarsay/JGroups,vjuranek/JGroups,rpelisse/JGroups,Sanne/JGroups,ibrahimshbat/JGroups,pruivo/JGroups,slaskawi/JGroups,rvansa/JGroups,slaskawi/JGroups,vjuranek/JGroups,deepnarsay/JGroups,rvansa/JGroups,ligzy/JGroups,belaban/JGroups,danberindei/JGroups,tristantarrant/JGroups,pruivo/JGroups,belaban/JGroups,rpelisse/JGroups,pferraro/JGroups,dimbleby/JGroups,TarantulaTechnology/JGroups,belaban/JGroups,kedzie/JGroups,ibrahimshbat/JGroups,pferraro/JGroups,danberindei/JGroups,rhusar/JGroups