as one of the (indirected) member variables has a user-declared move
ctor, this prevents the compiler from generating the default copy ctor
or assignment operator for the classes containing `timer`.
```
/home/kefu/dev/scylladb/utils/histogram.hh:440:5: warning: explicitly defaulted copy constructor is implicitly deleted [-Wdefaulted-function-deleted]
timed_rate_moving_average_and_histogram(const timed_rate_moving_average_and_histogram&) = default;
^
/home/kefu/dev/scylladb/utils/histogram.hh:437:31: note: copy constructor of 'timed_rate_moving_average_and_histogram' is implicitly deleted because field 'met' has a deleted copy constructor
timed_rate_moving_average met;
^
/home/kefu/dev/scylladb/utils/histogram.hh:298:17: note: copy constructor of 'timed_rate_moving_average' is implicitly deleted because field '_timer' has a deleted copy constructor
meter_timer _timer;
^
/home/kefu/dev/scylladb/utils/histogram.hh:212:13: note: copy constructor of 'meter_timer' is implicitly deleted because field '_timer' has a deleted copy constructor
timer<> _timer;
^
/home/kefu/dev/scylladb/seastar/include/seastar/core/timer.hh:111:5: note: copy constructor is implicitly deleted because 'timer<>' has a user-declared move constructor
timer(timer&& t) noexcept : _sg(t._sg), _callback(std::move(t._callback)), _expiry(std::move(t._expiry)), _period(std::move(t._period)),
```
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
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