TEMPORARY PUSH: Changing all cost calculation to be given in ms10.7-selectivity-old

- Added tests/check_costs.pl, a tool to verify optimizer cost calculations.
  - Most costs has been found with this program. All steps to calculate
    the new costs are documented in Docs/optimizer.costs
- User optimizer_cost variables are given in usec (as individual
  costs can be very small). Internally they are stored in ms.
- Changed DISK_READ_COST (was DISK_SEEK_BASE_COST) from a hard disk cost
  (9 ms) to common SSD cost (400MB/sec).
- Changed the following handler functions to return IO_AND_CPU_COST.
  This makes it easy to apply different cost modifiers in ha_..time()
  functions for io and cpu costs.
  - scan_time()
  - rndpos_time()
  - keyread_time()
- Enhanched keyread_time() to calculate the full cost of reading of a set
  of keys with a given number of ranges and optionall number of blocks that
  need to be accessed.
- Removed read_time() as keyread_time() + rndpos_time() is the same thing.
- Added the following new optimizer_variables:
  - optimizer_scan_lookup_cost
  - optimizer_row_lookup_cost
  - optimizer_index_lookup_cost
  - optimizer_disk_read_cost
- Added include/my_tracker.h  ; Useful include file to quickly test costs
  of a function.
- Tuned sequence and heap engine costs (rest will be done in an updated
  commit)

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