apache / iotdb / #9651

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[IOTDB-6072] Load: workaround to CPU 100% after loading tsfile when using JDK8 runtime (#10624) (#10625)

## Description
After loading tsfile, it causes 100% load on one CPU core.

## Reason
Root cause: high processor load for ScheduledThreadPoolExecutor with 0 core threads. It was a bug in JDK1.8.

See the following issues for more details:
[JDK-8129861](https://bugs.openjdk.java.net/browse/JDK-8129861)
[JDK-8022642](https://bugs.openjdk.java.net/browse/JDK-8022642)

## Solution
1. Set corePoolSize to 1 to avoid high processor load (a workaround)
2. Lazy initialization: init loadTsFileManager on first call (to avoid unnecessary threads' creation)

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