Added LruCache in envoy

source/common/lru_cache/BUILD

@@ -0,0 +1,17 @@
+load(
+    "//bazel:envoy_build_system.bzl",
+    "envoy_cc_library",
+    "envoy_package",
+)
+
+licenses(["notice"])  # Apache 2
+
+envoy_package()
+
+envoy_cc_library(
+    name = "lru_cache_lib",
+    hdrs = [
+        "lru_cache.h",
+        "lru_cache_inl.h",
+    ],
+)

source/common/lru_cache/lru_cache.h

@@ -0,0 +1,30 @@
+// For inclusion in .h files. The real class definition is in
+// `lru_cache_inl.h`.
+
+#pragma once
+
+#include <functional>
+
+#include "absl/container/flat_hash_map.h" // for hash<>
+
+namespace Envoy {
+namespace LruCache {
+
+namespace internal {
+template <typename T> struct SimpleLRUHash : public std::hash<T> {};
+} // namespace internal
+
+template <typename Key, typename Value, typename H = internal::SimpleLRUHash<Key>,
+          typename EQ = std::equal_to<Key>>
+class SimpleLRUCache;
+
+// Deleter is a functor that defines how to delete a Value*. That is, it
+// contains a public method:
+//  operator() (Value* value)
+// See example in the associated unittest.
+template <typename Key, typename Value, typename Deleter, typename H = internal::SimpleLRUHash<Key>,
+          typename EQ = std::equal_to<Key>>
+class SimpleLRUCacheWithDeleter;
+
+} // namespace LruCache
+} // namespace Envoy