Remove InitOnce from the port API.

This is not an API-breaking change, because it reduces the API that the
leveldb embedder must implement. The project will build just fine
against ports that still implement InitOnce.

C++11 guarantees thread-safe initialization of static variables inside
functions. This is a more restricted form of std::call_once or
pthread_once_t (e.g., single call site), so the compiler might be able
to generate better code [1]. Equally important, having less code in
port_example.h makes it easier to port to other platforms.

Due to the change above, this CL introduces a new approach for storing
the singleton BytewiseComparatorImpl instance returned by
BytewiseComparator(). The new approach avoids a dynamic memory
allocation, which eliminates the false positive from LeakSanitizer
reported in https://github.com/google/leveldb/issues/200

[1] https://stackoverflow.com/a/27206650/

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=212348004

CMakeLists.txt

@@ -149,6 +149,7 @@ target_sources(leveldb
     "${PROJECT_SOURCE_DIR}/util/logging.cc"
     "${PROJECT_SOURCE_DIR}/util/logging.h"
     "${PROJECT_SOURCE_DIR}/util/mutexlock.h"
+    "${PROJECT_SOURCE_DIR}/util/no_destructor.h"
     "${PROJECT_SOURCE_DIR}/util/options.cc"
     "${PROJECT_SOURCE_DIR}/util/random.h"
     "${PROJECT_SOURCE_DIR}/util/status.cc"
@@ -278,6 +279,7 @@ if(LEVELDB_BUILD_TESTS)
 
   leveldb_test("${PROJECT_SOURCE_DIR}/util/env_test.cc")
   leveldb_test("${PROJECT_SOURCE_DIR}/util/status_test.cc")
+  leveldb_test("${PROJECT_SOURCE_DIR}/util/no_destructor_test.cc")
 
   if(NOT BUILD_SHARED_LIBS)
     leveldb_test("${PROJECT_SOURCE_DIR}/db/autocompact_test.cc")