Make Cassandra Connector work with CommitLogTransfer better

In order to make Cassandra Connector work (better) with customized CommitLogTransfer, the following improvements/fixes are still needed:

• Fix commitLogTransferConfigs() in CassandraConnectorConfig.java  (Remove "commit.log.transfer" prefix from resulted config properties)
• Use AbstractDirectoryWatcher to detect new files in relocation directory in CommitLogPostProcessor.java
• Download error commitLogFiles only upon Cassandra CDC is started in CommitLogProcess.java
• Improve loggings in deleteCommitLog() in CommitLogUtil.java (Log file path instead of file name)