Move iterator state to instance object so they don't all share the same state
Move iterator state to instance object so they don't all share the same state

maxogden/node-leveldb,maxogden/node-leveldb,mikepb/node-leveldb,my8bird/node-leveldb,mikepb/node-leveldb,my8bird/node-leveldb,maxogden/node-leveldb,my8bird/node-leveldb