Produce SPV for a transaction

[dzhelezov]

Implement an interface (together with an API command) to produce Simple Payment Verification (SPV) for a given transaction. SPV serves as a trust-minized proof that the given transaction is accepted by the network.

An SPV proof for a transaction T will consist of 2/3+ hash paths in the DAG leading to 2/3+ milestones in the same round, indirectly confirming T. The transactions in the path may be either virtual or non-virtual.