Make Verkle instantiation easier

[acolytec3]

We have a few rough edges in how we initialize verkle trees at the moment. As long as you use the createVerkleTree constructor function, it's pretty straight forward. However, we should make a few changes:

• Have the constructor default to instantiate a MapDB if none is provided
• The _createRootNode function should be made public.
• We should just get rid of the database function all together. There's no need for an independent DB setter/getter as this should be part of the constructor process

[gabrocheleau]

Have some time this PM so will tackle this one