Summary:
Current commit cache size is 2^21. This was due to a type. With 2^23 commit entries we can have transactions as long as 64s without incurring the cost of having them evicted from the commit cache before their commit. Here is the math:
2^23 / 2 (one out of two seq numbers are for commit) / 2^16 TPS = 2^6 = 64s
Closes https://github.com/facebook/rocksdb/pull/3657

Differential Revision: D7411211

Pulled By: maysamyabandeh

fbshipit-source-id: e7cacf40579f3acf940643d8a1cfe5dd201caa35