kvserver: allow circuit-breaker to serve reads

Informs cockroachdb#74799. I hope it will ultimately close it, but not quite there
yet. I left extensive `TODO(during review)` comments.

Release note: None

pkg/kv/kvserver/BUILD.bazel

@@ -35,7 +35,6 @@ go_library(
         "replica_backpressure.go",
         "replica_batch_updates.go",
         "replica_circuit_breaker.go",
-        "replica_circuit_breaker_cancelstorage.go",
         "replica_closedts.go",
         "replica_command.go",
         "replica_consistency.go",
@@ -137,6 +136,7 @@ go_library(
         "//pkg/kv/kvserver/rditer",
         "//pkg/kv/kvserver/readsummary",
         "//pkg/kv/kvserver/readsummary/rspb",
+        "//pkg/kv/kvserver/spanlatch",
         "//pkg/kv/kvserver/spanset",
         "//pkg/kv/kvserver/split",
         "//pkg/kv/kvserver/stateloader",
@@ -224,7 +224,6 @@ go_test(
         "client_rangefeed_test.go",
         "client_relocate_range_test.go",
         "client_replica_backpressure_test.go",
-        "client_replica_circuit_breaker_bench_test.go",
         "client_replica_circuit_breaker_test.go",
         "client_replica_gc_test.go",
         "client_replica_test.go",