MVCC = true by default
MVCC = true by default

change nxt-default.properties to enable MVCC by default, to avoid
database lock timeout errors. MVCC is multiversion concurrency control,
good against db corruption.

burst-team/burstcoin,burst-team/burstcoin,burst-team/burstcoin,burst-team/burstcoin,burst-team/burstcoin