Cache proxies for sender/receiver pair

As the Ask_vs_tell_\* tests show, generating a proxy is slow, and since a proxy is unique to a sender/receiver pair, it could easily be cached in the `BackStage`. The cached proxy can be deleted when either the sender's or receiver's mailbox is shut down.

I.e. ask vs tell sequential without caching is

```
16k asks/s
13k tells/s
```

while with caching it's

```
16k asks/s
20k tells/s
```
