endpoint_filter() function assumes that each bucket of
std::unordered_multimap contains elements with the same key only, so
its size can be used to know how many elements with a particular key
are there. But this is not the case, elements with multiple keys may
share a bucket. Fix it by counting keys in other way.
Fixes#3229
Message-Id: <20190501133127.GE21208@scylladb.com>
(cherry picked from commit 95c6d19f6c)