Enable users to customize the directory path of cached datasets

[WenjieDu]

1. Feature description

Add a config file to record TSDB's cached dataset path and add functions to modify the path and move cached datasets to the specified directory.

2. Motivation

Currently, TSDB fixes its directory path of cached datasets as .tsdb_cached_datasets under user's home root. Due to caching all supported 168 datasets needs too large disk space, users may want to move tsdb's dataset path to other places, like a external disk.

3. Your contribution

Will make a PR.