Replace journal.log with an in place journal

Instead of writing the current head revision to the journal.log file we could make it an integral part of the node states: as OAK-3804 demonstrates we already have very good heuristics to reconstruct a lost journal. If we add the right annotations to the root node states this could replace the current approach. The latter is problematic as it relies on the flush thread properly and timely updating journal.log. See e.g. OAK-3303.