In order to prevent switching gtid_mode from ON_PERMISSIVE to ON when
there are ongoing anonymous transactions, we introduce a counter that
counts the number of ongoing anonymous transactions.

@atomic_class.h
- Auxiliary class to use C++ syntax to access atomic integers.

@rpl_gtid.h
- Add member functions in Gtid_state to acquire and release anonymous
ownership.

@rpl_gtid_execution.cc
- Acquire anonymous ownership when executing set gtid_next='anonymous'.

@rpl_gtid_state.cc
- Release anonymous ownership in update_gtid_impl.
- Acquire anonymous ownership in generate_automatic_gtid.

@binlog.cc:
- Decrease global counter when releasing anonymous ownership.
- When thread holds anonymous ownership, and gtid_end_transaction is
called, it should call update_on_commit instead of clear_owned_gtids, so
that it releases anonymous ownership.