[client] Cache the box shared key per remote peer in the Signal client (#7807)

encryption/sharedkey_limit.go

@@ -0,0 +1,8 @@
+//go:build !ios && !android
+
+package encryption
+
+// maxSharedKeys bounds the cache so peers that come and go (ephemeral peers get a
+// new key on every registration) cannot grow it for the lifetime of the process.
+// An entry costs about 130 bytes, so a full cache is around 8 MB.
+const maxSharedKeys = 1 << 16