vinyl: remove meaningless global stat counters

 - Remove tx and cursor latencies as they are useless - they actually
   account how long a tx/cursor was open, not latencies.
 - Remove vy_stat->get_latency as it doesn't account latency of select,
   besides we now have per index read latency.
 - Remove vy_stat->dumped_statements and dump_total as these statistics
   are reported per index as well.
 - VY_STAT_TX_OPS is currently unused (always 0). Let's use it for
   accounting the total number of statements committed in tx instead of
   vy_stat->write_count.