Fix for #2858 - db.revision() != headblocknum on start
Fix for #2858 - db.revision() != headblocknum on start

- abort pending changes prior to shutdown
- pop undo state on startup in case we crashed without aborting pending
- warn user when we load in this state as other things may be wrong

EOSIO/eos,EOSIO/eos,EOSIO/eos,EOSIO/eos,EOSIO/eos