Decrease reliance on VersionSet::NumberLevels()

Summary:
With column families VersionSet will not have a constant number of levels (each CF can have different options), so we'll need to eliminate call to VersionSet::NumberLevels()

This diff decreases number of callsites, but we're not there yet. It associates number of levels with Version (each version is associated with single CF) instead of VersionSet.

I have also slightly changed how VersionSet keeps track of manifest size.

This diff also modifies constructor of Compaction such that it takes input_version and automatically Ref()s it. Before this was done outside of constructor.

In next diffs I will continue to decrease number of callsites of VersionSet::NumberLevels() and also references to current_

Test Plan: make check

Reviewers: haobo, dhruba, kailiu, sdong

Reviewed By: sdong

Differential Revision: https://reviews.facebook.net/D15171
Move functions from VersionSet to Version

Summary:
There were some functions in VersionSet that had no reason to be there instead of Version. Moving them to Version will make column families implementation easier.

The functions moved are:
* NumLevelBytes
* LevelSummary
* LevelFileSummary
* MaxNextLevelOverlappingBytes
* AddLiveFiles (previously AddLiveFilesCurrentVersion())
* NeedSlowdownForNumLevel0Files

The diff continues on (and depends on) D15171

Test Plan: make check

Reviewers: dhruba, haobo, kailiu, sdong, emayanke

Reviewed By: sdong

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15183
Moving Compaction class to separate header file

Summary:
I'm sure we'll all agree that version_set.cc needs simplifying. This diff moves Compaction class to a separate file.

The diff depends on D15171 and D15183

Test Plan: make check

Reviewers: dhruba, haobo, kailiu, sdong

Reviewed By: kailiu

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15189
Move more functions from VersionSet to Version

Summary:
This moves functions:
* VersionSet::Finalize() -> Version::UpdateCompactionStats()
* VersionSet::UpdateFilesBySize() -> Version::UpdateFilesBySize()

The diff depends on D15189, D15183 and D15171

Test Plan: make check

Reviewers: kailiu, sdong, haobo, dhruba

Reviewed By: sdong

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15201
Remove the unnecessary use of shared_ptr

Summary:
shared_ptr is slower than unique_ptr (which literally comes with no performance cost compare with raw pointers).
In memtable and memtable rep, we use shared_ptr when we'd actually should use unique_ptr.

According to igor's previous work, we are likely to make quite some performance gain from this diff.

Test Plan: make check

Reviewers: dhruba, igor, sdong, haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15213