Add backend-jvstm-lf-cluster

  This is a clustered implementation based on the lock-free JVSTM (>= 2.0).
  It will duplicate some code already implemented in jvstm-cluster and
  jvstm-datagrid.  Nevertheless, it is expected that this backend may
  supersede the jvstm-cluster backend, thus eliminating the duplication.  We
  chose to 'inherit' from jvstm-common instead of to create a jvstm-lf-common
  backend, because it is not expectable for this module to fully supersede the
  jvstm-common module, in the near future.
[jvstm-lf] Rename RemoteCommit to CommitRequest

  A CommitRequest is sent by every (write) transaction that needs to commit.
  It contains the read set and write set of the transaction to commit.  After
  this, every node can locally continue its commits starting from this queue.
[jvstm-lf] Update POMs with dependency on JVSTM 2.0 (lock-free)
[jvstm-lf] Updates just to allow compilation with JVSTM 2
[jvstm-lf] Add a TransactionSignaller to access the default static SIGNALLER in JVSTM