Summary:
statistics are always concurrently updated regardless of whether a single instance or multiple instances are used. remove the warning since it's unnecessarily preventing people from sharing stats objects across rocksdb instances.
replace #1819 which was having some problem with our internal tools.
Closes https://github.com/facebook/rocksdb/pull/1826
Differential Revision: D4495970
Pulled By: ajkr
fbshipit-source-id: 6601f61
186c7eedb0