Provide statistics counters for MRR

We need statistics counters for MRR. In particular, we need

• How many MRR scans were performed
• How many key/rowid re-sort operations were made (Need separate counters for key and rowid ops).

The first attempt at counters was:

• Handler_mrr_init,
• Handler_mrr_extra_rowid_sorts,
• Handler_mrr_extra_key_sorts
  (see http://kb.askmonty.org/en/multi-range-read-optimization#counters for docs)

However, the names/semantics were deemed too complicated. Now, we need to come up with simpler definitions.

== Proposal#1 ==

• Handler_mrr_init - stays the same, counts rowid-ordered scans, key-ordered scans, and key-ordered rowid-ordered scans
• Handler_mrr_rowid_ordered_passes - counts number of { sort rowids; do a rowid-ordered read sweep; }

  passes

• Handler_mrr_key_ordered_passes - counts number of { sort keys; do a key-ordered read sweep; }

  passes

== Proposal#2 ==
Same as above but break Handler_mrr_init into three:

• Handler_mrr_key_ordered
• Handler_mrr_rowid_ordered
• Handler_mrr_key_rowid_ordered

== Decision ==

On the last optimizer call, we've decided to go with these counters:

Handler_mrr_init
Handler_mrr_key_refills
Handler_mrr_rowid_refills

and the first fill is not counted as a "refill".