test for issue_633
clear executed market transactions when we run out of depth
Fix crash: chain observer notifications should be asynchronous

Chain observers are free to yield if they wish, but nothing in the
push_block call stack should ever yield, so chain observer notifications
ought to be made asynchronously. This commit makes it so.
Fix wallet_backup_create
Update submodule
Rename COVER -> MARGIN in wallet history
Fix automatic wallet backups
Fix regression tests