Regular sync: pubsub subscriber for beacon blocks (#3220)

* add validation

* add block db check in validation

* merge

* in memory caching of seen blocks

* basic block processing

* Update BUILD.bazel

* use new receiveBlockNoPubsub

* fix build

* add TODO issue numbers

* add TODO issue numbers

* lint

shared/testutil/BUILD.bazel

@@ -9,6 +9,7 @@ go_library(
         "is_empty.go",
         "json_to_pb_converter.go",
         "log.go",
+        "random_bytes.go",
         "tempdir.go",
         "wait_timeout.go",
     ],

shared/testutil/random_bytes.go

@@ -0,0 +1,16 @@
+package testutil
+
+import (
+	"crypto/rand"
+	"testing"
+)
+
+// Random32Bytes generates a random 32 byte slice.
+func Random32Bytes(t *testing.T) []byte {
+	b := make([]byte, 32)
+	_, err := rand.Read(b)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return b
+}