The Binlog injector depends heavily on synching the
binlog injector thread and the mysqld clients by using
the injector_mutex and injector_cond.

When distributing schema changes, the function
ndbcluster_log_schema_op() handles the change distribution
across the mysqld, and it finally waits on events ack'ing
that all mysqld has acted upon the changes. This was
implemented as a wait for the NDB_SCHEMA_OBJECT::slock_bitmap
to be cleared, using the injector_cond / injector_mutex pair
in a pthread_condition wait.

It is the binlog injector thread which receive and handles
the events signaling the above ::slock_bitmap changes.
As a lock on the injector_mutex was also held when
polling the eventBuffer, where the injector thread spend
99.9...% of its time, it was hardly possible for the
pthread_cond signaling of ::slock_bitmap changes to get
through to ndbcluster_log_schema_op().

This patch introduce a condition variable in each
NDB_SCHEMA_OBJECT (NDB_SCHEMA_OBJECT::cond).
The above conditional wait for NDB_SCHEMA_OBJECT::slock_bitmap
to be cleared, is changed to wait on / signal
the NDB_SCHEMA_OBJECT::cond & ::mutex pair.