Print name of user comparator in LOG.

Summary:
The current code prints the name of the InternalKeyComparator
in the log file. We would also like to print the name of the
user-specified comparator for easier debugging.

Test Plan: make check

Reviewers: sheki

Reviewed By: sheki

CC: leveldb

Differential Revision: https://reviews.facebook.net/D11181

db/dbformat.cc

@@ -44,7 +44,8 @@ std::string InternalKey::DebugString(bool hex) const {
 }
 
 const char* InternalKeyComparator::Name() const {
-  return "leveldb.InternalKeyComparator";
+  return ("leveldb.InternalKeyComparator:" +
+          std::string(user_comparator_->Name())).c_str();
 }
 
 int InternalKeyComparator::Compare(const Slice& akey, const Slice& bkey) const {