some more small changes

The changes are:

• added comments to notify about slight problem: report for balance tracker for account_create[_with_delegation]_operation is incorrect prior to HF20 since fee was used to create initial vests on created account; however assuming null account is going to be properly cleared then leaving as is is better option than adding extra asset or even just a flag to account_created_operation
• stopped generation of vop from cancel_order call when used inside hardfork_hive_operation activity; it most likely never happened but better safe than sorry; if it did happen then from balance tracker perspective it would look like hardfork_hive_operation cleared account and then that account received assets from cancelled order which would lead to incorrect balance; we still have potential similar situation with HBD interest, but that would be much harder to correct if it actually happened
• turned off generation of block stats from p2p sync blocks; the stats provided were mostly empty (since there are no transactions during sync) and the parts that do give some information are not worth slowing down sync with excess logging
• added tests to cover generation of recently added escrow_approved_operation and escrow_rejected_operation