Simplify-iterator

[rameight]

This PR changes the way the Iteratee interface is defined. Previously, we had:

NewIteratorWithPrefix(prefix []byte)
NewIteratorWithStart(start []byte)

But no good way to get a prefixed iterator with a given start position. As that was missing, a lot of code made one of two mistakes:

1. Use NewIteratorWithStart( prefix ++ start). This solution is wrong; consider the prefix 0x0a and start 0x00. The returned iterator would also return e.g 0x0b, as it is after a in the keyspace.
2. Use NewIteratorWithPrefix( prefix ++ start). This is also wrong. Consider the prefix 0x0a and start 0x01. This iterator would miss a key at 0x0a02, since 0x0a01 is not a prefix to 0x0a02.

I replaced both of these with

NewIterator(prefix, start []byte)

[rameight]

@thientn @trinhdn97 The changes of Iterator in the PR #236 isn't necessary. This PR is an improvement of Iterator, please read description for more details.