This worklog adds the runtime side for the new FOR EXPORT
option for FLUSH TABLES <table_list>.

The FLUSH TABLES <table_list> FOR EXPORT statement:
- Acquires MDL_SHARED_NO_WRITE metadata lock on the tables
  listed in <table_list>. The effect is that FLUSH TABLES
  <table_list> FOR EXPORT will block until all active
  transactions that have updated any table in <table_list>
  have completed. While FLUSH TABLES <table_list> FOR EXPORT
  is in effect, no new updating transactions and no DDL
  statements are allowed. Read-only transactions will not be
  affected.
- Enters LOCK TABLES mode so that locks are held until
  the UNLOCK TABLES statement is executed.
- Notifies the storage engine by calling handler::extra()
  using the new flag HA_EXTRA_EXPORT. This allows the storage
  engine to perform any steps needed to make the table ready
  for export (e.g. quiesce the table and write metadata file).

The worklog also adds a new ha_table_flags() flag:
HA_CAN_EXPORT_TABLESPACE. This flags indicates that a given
storage engine support FLUSH TABLES <table_list> FOR EXPORT.
If such a statement includes tables for which the storage
engine does not have this flag, an error will be returned.