Publish milestone together with virtual transactions

[dzhelezov]

Currently a milestone is published without trunk and branch.
This issue should replace this with a milestone + virtual transaction as described in the spec

[cristina-vasiu]

A virtual transaction, is a non value transaction with the following differences from a normal non value transaction:

• signatureMessage -> contain milestone hash (for which this transaction is created)
• tag -> is used to save merkle index
• bundleNonce -> "1111111111111111111111111111111111111111111111111111111111111111" this is a hardcoded value to show that transaction is virtual
• address - is validator address (the one who created the milestone)
• bundle - null
• nonce - null
• branch, trunk (leaves from merkle tree)
• hash is computed with hash(branch || trunk)

These transactions are created when milestone is published if tips list is greater than 1. If there is only on tip, that one is also the merkle tree and it already exists in tangle.

For tips no virtual transaction are computed, there should be already transaction in tangle for them.
Right now all virtual transactions are attached to tangle, should be updated after 'Reconstruct virtual transactions locally #189'