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

util/coding.cc

@@ -6,7 +6,7 @@
 
 #include <algorithm>
 
-namespace leveldb {
+namespace rocksdb {
 
 void EncodeFixed32(char* buf, uint32_t value) {
 #if __BYTE_ORDER == __LITTLE_ENDIAN
@@ -298,4 +298,4 @@ uint64_t BitStreamGetInt(const Slice* src, size_t offset,
   return BitStreamGetInt(src->data(), src->size(), offset, bits);
 }
 
-}  // namespace leveldb
+}  // namespace rocksdb