Finalized vDHT approach

In order to finalize the required features to support the vDHT approach, following features have been implemented:
<ul>
<li>Data contains a prepare flag. The en/decoding has been modified for this.</li>
<li>Backend stores Data with prepre flag, but is not visible for get and digest calls.</li>
<li>New RPC "putConfirm" which can confirm or reject a temporary stored Data object.</li>
<li>New RPC "getLatest" returning all latest versions.</li>
<li>Added several tests checking above behaviour.</li>
</ul>