E3PC Decision Rule
If at least one DM aborted - decide abort.
If at least one DM committed - decide commit.
If IMAC and there is a quorum - move to Prepare-Commit.
If not IMAC and there is a quorum - move to Prepare-Abort.
Otherwise (i.e. no quorum) - Block
IMAC : a predicate that is true iff all the connected
members with max Last_attempt are in the PC state.