vinyl: use rmean for more stat counters

It's good to know not only the total number of events that have happened
since the server start, but also the average over the last few seconds.
So let's use rmean for more stat counters:

 - VY_STAT_GET: number of reads, including from cursor
   (former vy_stat->get)
 - VY_STAT_TX: number of transactions (former vy_stat->tx)
 - VY_STAT_TX_OPS: number of operations (read or write) issued by
   transactions (formerly, we had vy_stat->tx_stmts, which is the
   average number of operations per transaction)
 - VY_STAT_CURSOR: number of cursors used (former vy_stat->cursor)
 - VY_STAT_CURSOR_OPS: number of reads issued by cursors
   (former vy_stat->cursor_ops)

Also, drop get_read_disk and get_read_cache for now, as they are always
0. We might want to reintroduce them once we have read cache.