ANALYZE FORMAT=JSON: Show r_table_loops

ANALYZE FORMAT=JSON shows r_loops. This is the number of times the control was passed to this table.

However, this is not always equal to the number of times the table access method was invoked.

Reasons:

• Eq_ref lookup cache (studying it is the motivation for this MDEV)

• Any error in join_read_always_key()/cp_buffer_from_ref(). The most common
  kind of error is that we've got a NULL for the lookup key. (we were doing a ref access on t2.key=t1.col and we've got NULL for t1.col).

This is already done for eq_ref in 11.0, but not for other access methods.