Reset eol if we continue. Return NOTFOUND if checksum mismatch. WT-1840

[~michaelcahill] Please review. The fix specifically for the test failure is reseting eol if we read the next log file. I also added a fix so that if a checksum doesn't match and we asked specifically for a record we return WT_NOTFOUND (the case of a log cursor setting an invalid LSN and calling search()). I also bumped up the keys in test_cursor07.py that showed the problem.