This worklog extends the optimizer cost model to take into account
whether data and indexes must be read from disk or are likely already
present in a memory buffer.

-The cost model will use estimates from the handler/storage engines
 about how much of tables and indexes are present in memory (see
 WL#7168) in its cost calculations.

-A new cost constant named "memory_block_read_cost" representing the
 cost of accessing data that is in a memory buffer inside the storage
 engine. This constant corresponds to the existing cost constant
 "io_block_read_cost" that is used when a block needs to be read from
 disk.

In this initial version the default value for memory_block_read_cost
is the same as the default value for io_block_read_cost.

Per-file comments for some of the test changes:

main.index_merge_delete:
  One query now merges three indexes instead of two due to changes in the
  model for disk sweeps used by index merge.
main.index_merge_myisam:
  Minor change in cost estimates in JSON output due to changes to cost
  model for disk sweeps used by index merge.
main.innodb_explain_json_non_select_all
main.innodb_explain_json_non_select_none
main.innodb_explain_non_select_all
main.innodb_explain_non_select_none main.myisam_explain_json_non_select_all:
main.myisam_explain_json_non_select_none:
main.myisam_explain_non_select_all.result
myisam_explain_non_select_none.result
  Minor changes in cost estimates in JSON output due to changes to cost
  model for disk sweeps used by index merge and DS-MRR.
  Added more data to two queries to preserve original query plan as index
  merge.
main.partition_index_innodb:
  Increase number of records in table in order to preserve original query
  plan. This was needed due to changes in cost model for disk sweeps used
  by index merge.
main.subquery_sj_all:
main.subquery_sj_all_bka:
main.subquery_sj_all_bka_nixbnl:
main.subquery_sj_all_bkaunique:
  Two queries changes from using FirstMatch to use Materialization due
  to a small change in the cost estimate for DS-MRR.
opt_costmodel.result:
  Re-recorded to reflect that the engine_cost table now has a new entry
  for memory_block_read_cost.
opt_costmodel_flush.test:
opt_costmodel_flush.result:
  Replaced use of io_block_read_cost with memory_block_read_cost since
  the test tables are so small that they are in the InnoDB buffer.
  Changing the value for io_block_read_cost do not longer have any
  impact on the test queries.
opt_costmodel_restart.test:
opt_costmodel_restart.result:
  Replaced use of io_block_read_cost with memory_block_read_cost since
  the test tables are so small that they are in the InnoDB buffer.
  Changing the value for io_block_read_cost do not longer have any
  impact on the test queries.
opt_costmodel_warnings.result:
  Re-recorded to reflect that the engine_cost table now has a new entry
  for memory_block_read_cost.
suite/opt_trace/r/bugs_no_prot_all.result:
suite/opt_trace/r/bugs_ps_prot_all.result:
suite/opt_trace/r/range_no_prot.result:
suite/opt_trace/r/range_ps_prot.result:
  Minor changes in cost estimates optimizer trace due to changes to cost
  model for disk sweeps used by index merge and DS-MRR.
unittest/gunit/faketable.h:
  Initialize the handler object's table pointer to point to the table.
unittest/gunit/opt_costconstants-t.cc:
  Added unit tests for the new SE_cost_constants::memory_block_read_cost()
  function.
unittest/gunit/opt_costmodel-t.cc:
  Added unit tests for the new functions added to the Cost_model_table class.