Merkle db fix new return type

[dboehm-avalabs]

The New function on merkledb currently returns an unexported type instead of the exported interface. This PR fixes the return type and adds a test only new function that tests can use to access unexported fields.

[danlaine]

As it stands, the exported methods of merkleDB are still callable outside the merkledb package, so I don't think this really buys us anything. On the other hand, in avalanchego we do generally return interfaces rather than structs. We should probably have a larger discussion about whether we should return structs and accept interfaces in all of avalanchego.

@joshua-kim pointed me to this article and I think it makes reasonable points https://bryanftan.medium.com/accept-interfaces-return-structs-in-go-d4cab29a301b