Create backend jvstm-cluster

This backend implements the concept of a clustered JVSTM.  It extends from
jvstm-common.  Transactions are local to each node, but commits are
synchronized across all nodes.  For the moment, this will contain a (global)
lock-based commit implementation.
[jvstm-cluster] Extract cluster-related code from jvstm-ispn

Later, we'll implement here the support for a VBox cache.
Merge pull request #148 from smmf/feature/extract-jvstm-clustered

Feature/extract jvstm clustered