Summary:
As described in https://github.com/facebook/rocksdb/issues/563, we should add minor version to SONAME, since we break ABI with minor releases.

I also turned PLATFORM_SHARED_VERSIONED to true by default. This is true in LevelDB and it was switched to false by D15117 for no apparent reason. It should only be false for iOS.

Test Plan: `make shared_lib` produced librocksdb.dylib.3.10.0

Reviewers: sdong, yhchiang, meyering

Reviewed By: meyering

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D36573