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

util/ldb_cmd.cc

@@ -1024,7 +1024,7 @@ Status ReduceDBLevelsCommand::GetOldNumOfLevels(Options& opt,
   }
   int max = -1;
   for (int i = 0; i < versions.NumberLevels(); i++) {
-    if (versions.NumLevelFiles(i)) {
+    if (versions.current()->NumLevelFiles(i)) {
       max = i;
     }
   }