Bug#20561446 NDB : NDBEVENTBUFFER NEEDS MORE THREADING PROTECTION

Patch removes some sharing of data between threads, and also adds
some thread synchronization.

Application thread used "uninitialised" value of m_total_buckets as
indicator when to use a dummy schema trans blocking add and drop
nodes during subscribing to first event.  Now a new separate flag
is used for that.

Application thread was setting the inititial value for m_total_buckets
while it only was receiver that need to use it, this logic was moved to
receiver thread on SUB_START_CONF arrival.

Also the m_active_op_count member was written by application thread but
read by receiver, this was also moved to receiver on SUB_START_CONF,
SUB_STOP_CONF, and SUB_STOP_REF arrival.  Also purging event data when
all subscriptions was canceled was moved to happend immediately when it
occured, instead of waiting until next new subscription.

Synchronization was added around writing to m_ev_op.