- Avoid checking for has_transactions if killed flag is not checked
- Simplify code (Have checked with gcc -O3 that there is improvements)
- Added handler::fast_increment_statstics() to be used when a handler
  functions wants to increase two statistics for one row access.
- Made check_limit_rows_examened() inline (even if it didn't make any
  difference for gcc 7.5.0), still the right thing to do