Switch CRYPTO_BUFFER_POOL to SipHash-2-4.

This hash table, in applications that use pooling, can dedup received
certificates in memory and thus should use a keyed hash.

Change-Id: Idc40dc8f7463025183121642b30ea0de43ebac0e
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/51125
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/crypto/pool/internal.h b/crypto/pool/internal.h
index b39ee42..f9f4838 100644
--- a/crypto/pool/internal.h
+++ b/crypto/pool/internal.h
@@ -39,6 +39,7 @@
 struct crypto_buffer_pool_st {
   LHASH_OF(CRYPTO_BUFFER) *bufs;
   CRYPTO_MUTEX lock;
+  const uint64_t hash_key[2];
 };