Hotfix the bug in table cache's GetSliceForFileNumber

Forgot to fix this problem in master branch. Already fixed it in performance branch.
Automate the preparation step for a new release

Summary: Added a script that prepares the repo for facebook's new rocksdb release, which will automatically do some necessary work to make sure this repo is ready for 3rdparty release.

Test Plan:
Run this script and observed:

* new version was created (both in local and remote repo) as a git tag.
* build_version.cc was updated
* build_detect_platform was changed so that it won't create any new change.

Reviewers: haobo, dhruba, sdong, igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15003
Add clang-format rules

Summary:
The rule file is forked from that in Facebook's repo.

I'll add format file for now and team members can tune the rules later.

In this patch, I made only two changes in order to be consistent with existing coding style

`SpacesBeforeTrailingComments: 2`

`ColumnLimit:     80`

Test Plan: N/A

Reviewers: igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15015