Change namespace from leveldb to rocksdb

Summary:
Change namespace from leveldb to rocksdb. This allows a single
application to link in open-source leveldb code as well as
rocksdb code into the same process.

Test Plan: compile rocksdb

Reviewers: emayanke

Reviewed By: emayanke

CC: leveldb

Differential Revision: https://reviews.facebook.net/D13287

utilities/merge_operators/string_append/stringappend2.cc

@@ -12,7 +12,7 @@
 #include "rocksdb/merge_operator.h"
 #include "utilities/merge_operators.h"
 
-namespace leveldb {
+namespace rocksdb {
 
 // Constructor: also specify the delimiter character.
 StringAppendTESTOperator::StringAppendTESTOperator(char delim_char)
@@ -100,5 +100,5 @@ MergeOperators::CreateStringAppendTESTOperator() {
   return std::make_shared<StringAppendTESTOperator>(',');
 }
 
-} // namespace leveldb
+} // namespace rocksdb