message: Pass in leveldb::Slice into workers and not napi values
  This is more for consistency, there were variants on this. Also makes
  it more clear where the slices are created
diff: @@ -830,11 +830,11 @@ struct PutWorker : public BaseWorker {
  PutWorker (napi_env env,
  Database* database,
  napi_value callback,
  - napi_value key,
  - napi_value value,
  + leveldb::Slice key,
  + leveldb::Slice value,
  bool sync)
  : BaseWorker(env, database, callback, "leveldown.db.put"),
  - key_(ToSlice(env, key)), value_(ToSlice(env, value)) {
  + key_(key), value_(value) {
  options_.sync = sync;
  }
  @@ -859,8 +859,8 @@ NAPI_METHOD(db_put) {
  NAPI_ARGV(5);
  NAPI_DB_CONTEXT();
  - napi_value key = argv[1];
  - napi_value value = argv[2];
  + leveldb::Slice key = ToSlice(env, argv[1]);
  + leveldb::Slice value = ToSlice(env, argv[2]);
  bool sync = BooleanProperty(env, argv[3], "sync", false);
  napi_value callback = argv[4];
  @@ -877,11 +877,11 @@ struct GetWorker : public BaseWorker {
  GetWorker (napi_env env,
  Database* database,
  napi_value callback,
  - napi_value key,
  + leveldb::Slice key,
  bool asBuffer,
  bool fillCache)
  : BaseWorker(env, database, callback, "leveldown.db.get"),
  - key_(ToSlice(env, key)),
  + key_(key),
  asBuffer_(asBuffer) {
  options_.fill_cache = fillCache;
  }
  @@ -922,7 +922,7 @@ NAPI_METHOD(db_get) {
  NAPI_ARGV(4);
  NAPI_DB_CONTEXT();
  - napi_value key = argv[1];
  + leveldb::Slice key = ToSlice(env, argv[1]);
  napi_value options = argv[2];
  bool asBuffer = BooleanProperty(env, options, "asBuffer", true);
  bool fillCache = BooleanProperty(env, options, "fillCache", true);
  @@ -942,10 +942,10 @@ struct DelWorker : public BaseWorker {
  DelWorker (napi_env env,
  Database* database,
  napi_value callback,
  - napi_value key,
  + leveldb::Slice key,
  bool sync)
  : BaseWorker(env, database, callback, "leveldown.db.del"),
  - key_(ToSlice(env, key)) {
  + key_(key) {
  options_.sync = sync;
  }
  @@ -968,7 +968,7 @@ NAPI_METHOD(db_del) {
  NAPI_ARGV(4);
  NAPI_DB_CONTEXT();
  - napi_value key = argv[1];
  + leveldb::Slice key = ToSlice(env, argv[1]);
  bool sync = BooleanProperty(env, argv[2], "sync", false);
  napi_value callback = argv[3];
  @@ -1023,6 +1023,7 @@ NAPI_METHOD(db_approximate_size) {
  leveldb::Slice start = ToSlice(env, argv[1]);
  leveldb::Slice end = ToSlice(env, argv[2]);
  +
  napi_value callback = argv[3];
  ApproximateSizeWorker* worker = new ApproximateSizeWorker(env, database,
  