(Patch from Monty, slightly amended)

Fix rowid filtering optimization in best_access_path():

== Ref access + rowid filtering ==
The cost computations compare #records and index-only scan cost
(keyread_tmp) to find out the per-record advantage one will get if
they skip reading full table record.

The computations produce wrong result when:

- the #records are "clipped down" with s->worst_seeks or
  thd->variables.max_seeks_for_key. keyread_tmp is not clipped
  this way so the numbers are not comparable.

- access_factor is negative. This means index_only read is
  cheaper than non-index-only read.

This patch makes the optimizer not to consider Rowid Filtering in
such cases.
The decision is logged in the Optimizer Trace using
"rowid_filter_skipped" name.

== Range access + rowid filtering ==
when considering to use Rowid Filter with range access, do multiply
keyread_tmp by record_count. That way, it is comparable with the
range access's estimate, which is multiplied by record_count.