io_queue: Fix "delay" metrics
io_queue: Fix "delay" metrics

The delay value is initialized to 1s on start, which is not
true, and leads to a weird bug:

- graphana dashboard shows the maximum value out of all
  shards and all mountpoints
- compaction and memtable flush classes somehow get their
  classes initialized for "default" mountpoint too

On workloads where requests delay for less than 1 second,
graphana evaluates this metrics to 1.0sec most of the time
and draws not very informative horizontal line.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>
Message-Id: <20210212084733.20184-2-c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

syuu1228/seastar,cloudius-systems/seastar,cloudius-systems/seastar,avikivity/seastar,syuu1228/seastar,avikivity/seastar,scylladb/seastar,avikivity/seastar,scylladb/seastar,scylladb/seastar,syuu1228/seastar,cloudius-systems/seastar