gms: Add is_normal helper to endpoint_state
gms: Add is_normal helper to endpoint_state

It is faster than gossiper::is_normal because it avoids to do search in
the std::map<application_state, versioned_value>. It is useful for the
code in the fast path which needs to query if a node is in NORMAL
status.

Fixes #3500

Message-Id: <863e9f9aa49b36f12a8dac298be9c0d8c7d26065@scylladb.com>

duarten/scylla,scylladb/scylla,avikivity/scylla,duarten/scylla,avikivity/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla