This class exists for one purpose only: to serve as glue code between
dht::ring_position and boost::icl::interval_map. The latter requires
that keys in its intervals are:
* default constructible
* copyable
* have standalone compare operations
For this reason we have to wrap `dht::ring_position` in a class,
together with a schema to provide all this. This is
`compatible_ring_position`. There is one further requirement by code
using the interval map: it wants to do lookups without copying the
lookup key(s). To solve this, we came up with
`compatible_ring_position_or_view` which is a union of a key or a key
view + schema. As we recently found out, boost::icl copies its keys **a
lot**. It seems to assume these keys are cheap to copy and carelessly
copies them around even when iterating over the map. But
`compatible_ring_position_or_view` is not cheap to copy as it copies a
`dht::ring_position` which allocates, and it does that via an
`std::optional` and `std::variant` to add insult to injury.
This patch make said class cheap to copy, by getting rid of the variant
and storing the `dht::ring_position` via a shared pointer. The view is
stored separately and either points to the ring position stored in the
shared pointer or to an outside ring position (for lookups).
Fixes: #11669Closes#11670
169a8a66f2