producer/request_state: make set_value() idempotent

In some racy situations it may happen that the request is already
errored out. Consider the following sequence of actions.

replicate_f - succeeded but set_value() not called
-- scheduling point --
term change -> sync() -> GC of inflight requests, request is marked
timedout

now set_value() is called in the original fiber, this triggers an
assert.

Relaxing the assert condition to make it idempotent. Subsequent client
retry of the request will be marked success (once the change is applied
in the stm and the request state is populated).

Unable to reproduce in a unit test mainly due to lack of an idempotent
client in the unit test fixture.

src/v/cluster/producer_state.cc

@@ -32,12 +32,10 @@ result_promise_t::future_type request::result() const {
 }
 
 void request::set_value(request_result_t::value_type value) {
-    vassert(
-      _state <= request_state::in_progress && !_result.available(),
-      "unexpected request state during result set: {}",
-      *this);
-    _result.set_value(value);
-    _state = request_state::completed;
+    if (_state != request_state::completed) {
+        _result.set_value(value);
+        _state = request_state::completed;
+    }
 }
 
 void request::set_error(request_result_t::error_type error) {