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

encryption/sharedkey_limit_mobile.go

@@ -0,0 +1,8 @@
+//go:build ios || android
+
+package encryption
+
+// maxSharedKeys is small on mobile, where the process runs under a tight memory
+// limit. A miss only costs a fresh key derivation. An entry costs about 130 bytes,
+// so a full cache is around 130 KB.
+const maxSharedKeys = 1 << 10