Improvements:

* Cache the File objects to reduce the size of TsFileInsertionEvents.

* Limit the frequency of flushing TsFiles when cocurrently creating historical extractors.

* If cluster heartbeat doesn't need PipeMetaList, do not get into PipeTaskAgent. This will prevent datanode from turning UNKNOWN when many pipes are being created.

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Co-authored-by: NSteve Yurong Su <rong@apache.org>