Before the patch, dht::sharder could be instantiated and it would
behave like a static sharder. This is not safe with regards to
extensions of the API because if a derived implementation forgets to
override some method, it would incorrectly default to the
implementation from static sharder. Better to fail the compilation in
this case, so extract static sharder logic to dht::static_sharder
class and make all methods in dht::sharder pure virtual.
This also allows us to have algorithms indicate that they only work
with static sharder by accepting the type, and have compile-time
safety for this requirement.
schema::get_sharder() is changed to return the static_sharder&.
9da3bd84c7