Add possibility to store nodes of the tree (MapStore)

[liamsi]

Also reuse stored hashes instead of recomputing them each time.

Copying the related review comment here:

There are other changes that we may need to make to the underlying merkletree implementation that may not be mergeable upstream. For example, we should probably change the implementation to store key=>value pairs of nodes in the tree using the same interface as the SMT MapStore, so that we can plug in other MapStores like an IPFS-based one, and reuse the same code: https://github.com/lazyledger/smt-ipfs-mapstore

This is something that needs to be investigated a bit though, as from my understanding the NebulousLabs merkletree library I think may compute the root on demand and doesn't keep around any of the old nodes in memory. Alternatively, it may be possible to implement this MapStore storage functionality within the NMT library, which uses the MapStore as "cache", and only invokes the NebulousLabs merkletree library to recompute roots if the node data is unavailable in the MapStore.

related to: #6

[liamsi]

#16 is sufficient to store the nodes on IPFS. Will close this one for now. There is another issue about storing the inner nodes in memory here: #17.