span: add MultiFrontier data structure

This patch adds a new `MultiFrontier` data structure that can be used
when we want a frontier that's partitioned into sub-frontiers under
the hood based on a custom partitioner function.

Release note: None

Author: andyyang890

pkg/util/span/BUILD.bazel

@@ -6,6 +6,7 @@ go_library(
     srcs = [
         "doc.go",
         "frontier.go",
+        "multi_frontier.go",
         "test_helper.go",
         ":btreefrontierentry_interval_btree.go",  #keep
     ],
@@ -16,6 +17,7 @@ go_library(
         "//pkg/roachpb",
         "//pkg/util/buildutil",
         "//pkg/util/container/heap",
+        "//pkg/util/container/heaputil",
         "//pkg/util/hlc",
         "//pkg/util/interval",
         "//pkg/util/syncutil",
@@ -29,6 +31,7 @@ go_test(
     srcs = [
         "frontier_fuzz_test.go",
         "frontier_test.go",
+        "multi_frontier_test.go",
         ":btreefrontierentry_interval_btree_test.go",  #keep
     ],
     data = glob(["testdata/**"]),
@@ -37,10 +40,12 @@ go_test(
         "//pkg/roachpb",
         "//pkg/util/container/heap",
         "//pkg/util/hlc",
+        "//pkg/util/iterutil",
         "//pkg/util/leaktest",
         "//pkg/util/log",
         "//pkg/util/randutil",
         "//pkg/util/timeutil",  #keep
+        "@com_github_cockroachdb_errors//:errors",
         "@com_github_stretchr_testify//require",
     ],
 )