Some fixes to BackupableDB

Summary:
(1) Report corruption if backup meta file has tailing data that was not read. This should fix: https://github.com/facebook/rocksdb/issues/81 (also, @sdong reported similar issue)
(2) Don't use OS buffer when copying file to backup directory. We don't need the file in cache since we won't be reading it twice
(3) Don't delete newer backups when somebody tries to backup the diverged DB (restore from older backup, add new data, try to backup). Rather, just fail the new backup.

Test Plan: backupable_db_test

Reviewers: ljin, dhruba, sdong

Reviewed By: ljin

CC: leveldb, sdong

Differential Revision: https://reviews.facebook.net/D16287
make key evenly distributed between 0 and FLAGS_num

Summary:
The issue is that when FLAGS_num is small, the leading bytes of the key
are padded with 0s. This makes all keys have the same prefix 00000000

Most of the changes are just to make lint happy

Test Plan: ran db_bench

Reviewers: sdong, haobo, igor

Reviewed By: sdong

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16317
Fix a buggy assert

Summary: The assert was pointless since if if prefix is the same as the whole key, assertion will surely fail. Reason behind is when performing the internal key comparison, if user keys are the same, *key with smaller transaction id* wins.

Test Plan: make -j32 && make check

Reviewers: sdong, dhruba, haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16551