Summary:
Current implementation holds on to 10% of snapshots for 10x longer, and 1% of snapshots 100x longer.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6171

Test Plan:
```
make -j32 crash_test

Differential Revision: D19038399

Pulled By: maysamyabandeh

fbshipit-source-id: 75da2dbb5c47a0b3f37d299b8719e392b73b42c0