unconst is a small help that converts a const iterator to a non-const
iterator with the help of the container. Currently it is using the
boost iterator/range libraries.
Convert it to <ranges> as part of an effort to standardize on a single
range library. Its only user in mutation_partition is converted as well.
Due to more iteroperability problems between <range> and boost, some
calls to boost::adaptors::reversed have to be converted as well.
Intended to be used in memtable/cache, as opposed to the old
mutation_partition which will be intended to be used as temporary
object.
The two will have different trade-offs regarding memory efficiency and
algorithms.
In this commit there is no change in logic, the class is mostly
copied. Some methods which are not needed on the v2 model were removed
from the interface.
Logic changes will be introduced in later commits.
