This patch implements I_S.PARTITIONS as a system view over data
dictionary tables, and remove 5.7 code that uses a temporary
table to represent I_S view.

Following changes are made in the patch:

* Define a system view over data dictionary tables representing
  I_S.PARTITIONS.

* Add following new dictionary columns to store utf8 string version.

  mysql.table_partitions.description_utf8 representing
  I_S.PARTITIONS.DESCRIPTION

  mysql.tables.subpartition_expression_utf8 representing
  I_S.PARTITIONS.SUBPARTITION_EXPRESSION

  mysql.tables.partition_expression_utf8 representing
  I_S.PARTITIONS.PARTITION_EXPRESSION

* Make dd::info_schema::Statistics_cache implementation to
  consider not just schema_name and table_name for cache key, but
  also include partition name. This is required now because we
  now cache per partition dynamic statistics with this WL.

* Make dd::info_schema::Statistics_cache implementation to cache
  the table checksum value. This value is computed only for
  partitions.

* Make implementation of Statistics_cache::read_stat_by_open_table()
  to fetch per partition dynamic table statistics.

* If a partition is not found when fetching dynamic table
  statistics, then we will report a warning and continue
  processing new rows.  This might happen when I_S system view
  execution starts and later the partition table is ALTERed. A
  test case parts.partition_debug_sync_innodb is updated to cover
  this scenario.

* Change definition of internal sql functions to accept partition
  name.This is used to calculate the per partition statistics.

  We currently read the statistics for partitioned table by
  opening the table. We can probably improve the performance for
  InnoDB partitioned tables, by calling SE api's that read just
  the statistics. This is not done as of now.

* Introduce internal sql function
  INTERNAL_GET_PARTITION_NODEGROUP() to fetch
  I_S.PARTITIONS.NODEGROUP value from
  mysql.table_partitions.options.

* There is no consumer of function expr_to_string() requesting
  the string in hex form. So, the patch modifies it to return
  only string.

* Remove 5.7 code from sql_show.cc for I_S.PARTITIONS.

* Added ORDER BY clauses in some of I_S query to force order of
  tuple returned. Because, now the optimizer returns the rows and
  not read from temporary table as in 5.7 I_S design.

* Remove part of test case in main.information_schema. Because,

  - The problem reported back then is only applicable for 5.7
    code.

  - The output of EXPLAIN ... <select on I_S system view> goes
    through optimizer and then plan return might vary.

* We now show 1024 character of partition comment string, unlike just 80
  characters shown in 5.7 by I_S.