Make memDB iterators not use channels

[daniel-farina]

Using channels in the memdb iterator is very high overhead, that doesn't make sense imo: https://github.com/tendermint/tm-db/blob/release/0.6.x/memdb_iterator.go#L40-L85

This should instead all be put into a struct, which would make this performance sensitive code way faster.

tendermint/tm-db#188