broker: size List response chunks to fit gRPC's 32KB buffer pool tier

Reduce maxJournalsPerListResponse from 1000 to 25 so that marshaled
ListResponse messages stay within the 32KB gRPC buffer pool tier.

At typical journal sizes (~750-1300 bytes per ListResponse_Journal), 25
journals produces messages of ~19-32KB. Previously, larger chunks
produced messages > 32 KB, landing in the 1MB buffer pool tier. When
many List watch streams sent responses concurrently, this caused
significant memory spikes from pooled 1MB buffers.

Author: williamhbaker

broker/list_apply_api.go

@@ -277,9 +277,8 @@ func (svc *Service) Apply(ctx context.Context, claims pb.Claims, req *pb.ApplyRe
 // * In large deployments, responses would bump against gRPC maximum messages sizes.
 // * Unary responses can't represent a long-lived watch.
 //
-// List was updated to become a server-streaming API which addresses both issues,
-// while remaining compatible with older clients that expect a unary API,
-// so long as the entire response set can be sent as a single message.
-// This value is large to allow older deployments to continue to function,
-// without being SO large that we bump against gRPC message limits.
-var maxJournalsPerListResponse = 1000
+// List was updated to become a server-streaming API which addresses both issues.
+//
+// This value is sized to keep marshaled ListResponse messages within the 32KB
+// gRPC buffer pool tier for typically sized journal specs.
+var maxJournalsPerListResponse = 25