- Before any multiple add() calls, always use (if trace_started()).
- Add unlikely() around all tests of trace_started().
- Change trace.add(); trace.add(); to trace.add().add();
- When trace.add() goes over several line, use the following formating:
trace.
 add(xxx).
 add(yyy).
 add(zzz);

This format was choosen after a discussion between Sergei Petrunia and
me as it looks similar indepedent if 'trace' is an object or a
pointer. It also more suitable for an editors auto-indentation.

Other things:

Added DBUG_ASSERT(thd->trace_started()) to a few functions that should
only be called if trace is enabled.

"use_roworder_index_merge: true" changed to "use_sort_index_merge: false"
As the original output was often not correct.
Also fixed the related 'cause' to be correct.

In best_access_path() print the cost (and number of rows) before
checking if it the plan should be used. This removes the need to print
the cost in two places.

Changed a few "read_time" tags to "cost".