New ldb command to convert compaction style

Summary:
Add new command "change_compaction_style" to ldb tool. For
universal->level, it shows "nothing to do". For level->universal, it
compacts all files into a single one and moves the file to level 0.

Also add check for number of files at level 1+ when opening db with
universal compaction style.

Test Plan:
'make all check'. New unit test for internal convertion function. Also manully test various
cmd like:

./ldb change_compaction_style --old_compaction_style=0
--new_compaction_style=1 --db=/tmp/leveldbtest-3088/db_test

Reviewers: haobo, dhruba

Reviewed By: haobo

CC: vamsi, emayanke

Differential Revision: https://reviews.facebook.net/D12603

include/rocksdb/db.h

@@ -209,9 +209,10 @@ class DB {
   // after compaction is reduced, that level might not be appropriate for
   // hosting all the files. In this case, client could set reduce_level
   // to true, to move the files back to the minimum level capable of holding
-  // the data set.
+  // the data set or a given level (specified by non-negative target_level).
   virtual void CompactRange(const Slice* begin, const Slice* end,
-                            bool reduce_level = false) = 0;
+                            bool reduce_level = false,
+                            int target_level = -1) = 0;
 
   // Number of levels used for this DB.
   virtual int NumberLevels() = 0;