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/utilities/stackable_db.h

@@ -104,8 +104,9 @@ class StackableDB : public DB {
   }
 
   virtual void CompactRange(const Slice* begin, const Slice* end,
-                            bool reduce_level = false) override {
-    return sdb_->CompactRange(begin, end, reduce_level);
+                            bool reduce_level = false,
+                            int target_level = -1) override {
+    return sdb_->CompactRange(begin, end, reduce_level, target_level);
   }
 
   virtual int NumberLevels() override {