test: add more tests and exception helper
fix(stats): STAT line always has 3 parts
feat(shading): prepare the interface for hashing

Add the default hash function used by EnyimMemcached.
Client() ctor now accepts address in tuple, like socket module.
feat(-multi_get): drop multi_get/Kestrel support

There is no need to support both client-side shading and Kestrel,
and client-side shading makes multi_get less useful.