Support to unload FST's .tip into memory,make load or unload configuable!. [LUCENE-7826]

in real case,we use lucene index many documents. But some machine have not much memory.,once documents reach up to tens of billion,lucene can not start because of no enough memory. Most of the memry cost is FST;s .tip content.
So I want to pull my change on lucene core to make load FST's .tip into memory become configurable!
What do you think?

---
Migrated from [LUCENE-7826](https://issues.apache.org/jira/browse/LUCENE-7826) by Xibao, updated Jan 30 2019
Linked issues:
- #9681
- #8885

Start by reviewing the Lucene core code responsible for loading FST .tip content into memory, then read linked issues #9681 and #8885 for related context. The issue is complete when the loading or unloading behavior is configurable and the memory-constrained startup case is addressed, with appropriate validation identified during investigation.

Written by the indexing model from the issue text.