WL#7083 step 5.2. Anonymous transactions: User-visible changes and tests for ANONYMOUS_TRANSACTION_COUNT.

@sql/mysqld.cc
- Introduce status variable ANONYMOUS_TRANSACTION_COUNT.

@sql/rpl_gtid_execution.cc
- Assert that global counter is positive when thread holds anonymous ownership.

@sql/sys_vars.h
- Make @@SESSION.GTID_OWNED contain "ANONYMOUS" when thread holds anonymous ownership.
- Generate error when changing gtid_mode to ON if there are ongoing
anonymous transactions.
- Generate error when changing gtid_mode to OFF if there are ongoing
GTID-transactions.

@mysql-test/include/execute_at_sync_point.inc
- Split this into two files: execute_to_sync_point.inc and
execute_from_sync_point.inc

@mysql-test/suite/binlog/t/binlog_anonymous_ownership.test
- New test case to test anonymous ownership.

@sql/sql_parse.cc
- New sync point used by binlog_anonymous_ownership.test.

@mysql-test/suite/binlog/t/binlog_gtid_next_anonymous_implicit_commit.test
- This tiny test is now one of the cases tested in
  binlog_anonymous_ownership.test, hence we can remove this file.