[RocksDB] Preallocate new MANIFEST files

Summary: We don't preallocate MANIFEST file, even though we have an option for that. This diff preallocates manifest file every time we create it

Test Plan: make check

Reviewers: dhruba, haobo

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17163