Fix Mac compile error: C++11 forbids default arguments for lambda expressions

Summary:
Fix the following Mac compile error.
db/db_test.cc:8686:52: error: C++11 forbids default arguments for lambda expressions [-Werror,-Wlambda-extensions]
  auto gen_l0_kb = [this](int start, int size, int stride = 1) {
                                                   ^        ~
Test Plan:
db_test
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
Sanitize block-based table index type and check prefix_extractor

Summary:
Respond to issue reported
https://www.facebook.com/groups/rocksdb.dev/permalink/651090261656158/
Change the Sanitize signature to take both DBOptions and CFOptions

Test Plan: unit test

Reviewers: sdong, yhchiang, igor

Reviewed By: igor

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D25041
remove checking lower bound of level size

Summary:
as title

Test Plan:
make db_test