Persistent MerkleTree storage

[ed255]

Our MerkleTree implementation uses memory to store the leaves. To store the MerkleTree we need to serialize it entirely.

For applications where the MerkleTree is big and persistent storage is needed, this is very inconvenient.

A possible solution for this would be that the MerkleTree uses a generic key-value storage, and we can plug in a memory or persistent storage implementation when needed.

This change may affect many places of the code base.