c api: optimize uniq raw ptr wrapper

c api wrap many rocksdb ptr types, such as rocksdb::Iterator is warpped as:

  struct rocksdb_iterator_t {
    Iterator* rep;
  };

this introduced an indirect ptr access, making it slow, esp. ToplingDB
is very fast such as iter->key() latency is less than 5 ns, this overhead
is significant compared to real work.

In this commit, such warps are optimized out, `rocksdb::Iterator*` is
converted to and from `rocksdb_iterator_t*`, to minimize code changes,
we add template class ObjectAtHere as a place holder to emulate `rep`'s
behavior.

Not all such ptrs can be safely optimized, other similar wrapped ptrs are:
     1. DB*
     2. ColumnFamilyHandle*
     3. TransactionDB*
     4. OptimisticTransactionDB*

Author: rockeet