The problem was that TRUNCATE TABLE didn't take a exclusive
lock on a table if it resorted to truncating via delete of
all rows in the table. Specifically for InnoDB tables, this
could break proper isolation as InnoDB ends up aborting some
granted locks when truncating a table.

The solution is to take a exclusive metadata lock before
TRUNCATE TABLE can proceed. This guarantees that no other
transaction is using the table.

Incompatible change: Truncate via delete no longer fails
if sql_safe_updates is activated (this was a undocumented
side effect).

libmysqld/CMakeLists.txt:
  Add new files to the build list.
libmysqld/Makefile.am:
  Add new files to the build list.
mysql-test/extra/binlog_tests/binlog_truncate.test:
  Add test case for Bug#42643
mysql-test/include/mix1.inc:
  Update test case as TRUNCATE TABLE now grabs a exclusive lock.
  Ensure that TRUNCATE waits for granted locks on the table.
mysql-test/suite/binlog/t/binlog_truncate_innodb.test:
  As with other data modifying statements, TRUNCATE is still not
  possible in a transaction with isolation level READ COMMITTED
  or READ UNCOMMITED. It would be possible to implement so, but
  it is not worth the effort.
mysql-test/suite/binlog/t/binlog_truncate_myisam.test:
  Test under different binlog formats.
mysql-test/suite/binlog/t/disabled.def:
  Re-enable test case.
mysql-test/t/innodb_bug38231.test:
  Truncate no longer works with row-level locks.
mysql-test/t/mdl_sync.test:
  Ensure that a acquired lock is not given up due to a conflict.
mysql-test/t/partition_innodb_semi_consistent.test:
  End transaction as to release metadata locks.
mysql-test/t/truncate.test:
  A metadata lock is now taken before the object is verified.
sql/CMakeLists.txt:
  Add new files to the build list.
sql/Makefile.am:
  Add new files to the build list.
sql/datadict.cc:
  Introduce a new file specific for data dictionary operations.
sql/datadict.h:
  Add header file.
sql/sql_base.cc:
  Rename data dictionary function.
sql/sql_bitmap.h:
  Include dependency.
sql/sql_delete.cc:
  Move away from relying on mysql_delete() to delete all rows of
  a table. Thus, move any bits related to truncate to sql_truncate.cc
sql/sql_delete.h:
  Remove parameter.
sql/sql_parse.cc:
  Add protection against the global read lock -- a intention
  exclusive lock can be acquired in the truncate path.
sql/sql_show.cc:
  Add sync point for testing scenarios where a pending flush
  is ignored.
sql/sql_truncate.cc:
  Acquire a shared metadata lock before accessing table metadata.
  Upgrade the lock to a exclusive one if the table can be re-created.
  Rework binlog rules to better reflect the requirements.
sql/sql_yacc.yy:
  Set appropriate lock types for table to be truncated.
sql/table.h:
  Move to data dictionary header.