Use erlang:whereis/1 for checking if a store is running

A store's process uses its `StoreId` as its registered name. This is a
public interface of Ra so we can depend on it. Reading from key metrics
counters is already very fast but switching to `whereis/1` eliminates
basically all overhead of this function. When used heavily (for example
in RabbitMQ while publishing and consuming rapidly) the CPU time spent
on `is_store_running/1` disappears from the output of a perf recording
and a flamegraph.

Author: the-mikedavis

src/khepri_cluster.erl

@@ -1576,11 +1576,7 @@ get_store_ids() ->
 %% @doc Indicates if `StoreId' is running or not.
 
 is_store_running(StoreId) ->
-    ThisNode = node(),
-    RaServer = khepri_cluster:node_to_member(StoreId, ThisNode),
-    Timeout = khepri_app:get_default_timeout(),
-    KeyMetrics = ra:key_metrics(RaServer, Timeout),
-    Runs = maps:get(state, KeyMetrics) =/= noproc,
+    Runs = erlang:whereis(StoreId) =/= undefined,
 
     %% We know the real state of the Ra server. In the case the Ra server
     %% stopped behind the back of Khepri, we update the cached list of running