Enable push-based shuffle when multiple app attempts are enabled and manage concurrent access to the state in a better way

In the current implementation of RemoteBlockPushResolver, two ConcurrentHashmap are used to store #1 applicationId -> mergedShuffleLocalDirPath #2 applicationId+attemptId+shuffleID -> mergedShuffleParitionInfo. As there are four types of messages: ExecutorRegister, PushBlocks, FinalizeShuffleMerge and ApplicationRemove, will trigger different types of operations within these two hashmaps, it is required to maintain strong consistency about the informations stored in these two hashmaps. Otherwise, either there will be data corruption/correctness issues or memory leak in shuffle server. 

We should come up with systematic way to resolve this, other than spot fixing the potential issues.