c/producer_state: keep producer inflight requests queue bounded

Idempotent producers are allowed to produce up to 5 at a time, the five
requests outcome is cached in the producer for deduplication. Previously
the `_inflight_requests` list might grow unbounded if state machine
apply was lagging. This causes performance issues and unnecessary memory
pressure. Since the max inflight requests count is limited by producer
it is enough to keep only last 5 inflight requests in the
`_inflight_requests` list.

Signed-off-by: Michał Maślanka <michal@redpanda.com>

src/v/cluster/producer_state.cc

@@ -187,6 +187,17 @@ result<request_ptr> requests::try_emplace(
     // All invariants satisfied, enqueue the request.
     _inflight_requests.emplace_back(
       ss::make_lw_shared<request>(first, last, current, result_promise_t{}));
+    // if there are more than max cached requests inflight start dropping the
+    // oldest of them
+    while (_inflight_requests.size() > requests_cached_max
+           && _inflight_requests.front()->has_completed()) {
+        _inflight_requests.pop_front();
+        // clear finished requests as the producer will not be interested in
+        // them anymore
+        if (!_finished_requests.empty()) {
+            _finished_requests.clear();
+        }
+    }
 
     return _inflight_requests.back();
 }