gms: Optimize gossiper::is_alive
gms: Optimize gossiper::is_alive

In perf-flame, I saw in

service::storage_proxy::create_write_response_handler (2.66% cpu)

  gossiper::is_alive takes 0.72% cpu
  locator::token_metadata::pending_endpoints_for takes 1.2% cpu

After this patch:

service::storage_proxy::create_write_response_handler (2.17% cpu)

  gossiper::is_alive does not show up at all
  locator::token_metadata::pending_endpoints_for takes 1.3% cpu

There is no need to copy the endpoint_state from the endpoint_state_map
to check if a node is alive. Optimize it since gossiper::is_alive is
called in the fast path.

Message-Id: <a4bee698fbf6bd8b0147a507f2072211cf92ff62@scylladb.com>

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