Correction of quorum logic for 1 and 2 node clusters

* Removed the wrong rule that at least 3 nodes are required
* The rule is simply N/2 + 1 (majority of nodes)
* 1 node cluster => local write is enough
* 2 node cluster => local + the other
Merge pull request #52 from patriknw/wip-44-quorum-patriknw

Correction of quorum logic for 1 and 2 node clusters