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VersionSet::Get(): Bring back the logic of skipping key range check when there are <=3 level 0 files

Summary:
https://reviews.facebook.net/D17205 removed the logic of skipping file key range check when there are less than 3 level 0 files. This patch brings it back.

Other than that, add another small optimization to avoid to check all the levels if most higher levels don't have any file.

Test Plan: make all check

Reviewers: ljin

Reviewed By: ljin

Subscribers: yhchiang, igor, haobo, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D19035
Fix a bug causing LOG is not created when max_log_file_size is set.

Summary:
Fix a bug causing LOG is not created when max_log_file_size is set.
This bug is reported in issue #174.

Test Plan:
Add TEST(AutoRollLoggerTest, LogFileExistence).
make auto_roll_logger_test
./auto_roll_logger_test

Reviewers: haobo, sdong, ljin, igor, igor2

Reviewed By: igor2

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D19053
Refactor: group metadata needed to open an SST file to a separate copyable struct

Summary:
We added multiple fields to FileMetaData recently and are planning to add more.
This refactoring separate the minimum information for accessing the file. This object is copyable (FileMetaData is not copyable since the ref counter). I hope this refactoring can enable further improvements:

(1) use it to design a more efficient data structure to speed up read queries.
(2) in the future, when we add information of storage level, we can easily do the encoding, instead of enlarge this structure, which might expand memory work set for file meta data.

The definition is same as current EncodedFileMetaData used in two level iterator, so now the logic in two level iterator is easier to understand.

Test Plan: make all check

Reviewers: haobo, igor, ljin

Reviewed By: ljin

Subscribers: leveldb, dhruba, yhchiang

Differential Revision: https://reviews.facebook.net/D18933
Include max_write_buffer_number >= 2 to SanitizeOptions.

Summary: Include max_write_buffer_number >= 2 to SanitizeOptions.

Test Plan: make all check

Reviewers: haobo, sdong, igor, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19077