Speed up DB::Open() and Version creation by limiting the number of FileMetaData initialization.

Summary:
This diff speeds up DB::Open() and Version creation by limiting the number of FileMetaData initialization. The behavior of Version::UpdateAccumulatedStats() is changed as follows:

* It only initializes the first 20 uninitialized FileMetaData from file.  This guarantees the size of the latest 20 files will always be compensated when they have any deletion entries.  Previously it may initialize all FileMetaData by loading all files at DB::Open().
* In case none the first 20 files has any data entry, UpdateAccumulatedStats() will initialize the FileMetaData of the oldest file.

Test Plan: db_test

Reviewers: igor, sdong, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D24255