Support lack of key generated on getrecipientsharedkey and receiver

[realvarx]

Is your feature request related to a problem? Please describe.

If the shared_key which has to be generated by the other atSign hasn't been generated by the moment that getrecipientsharedkey is called, it won't be possible to decrypt the messages as that situation isn't handled. That leaves the user with no other option that stopping and running again the client.

Describe the solution you'd like

Create a global variable with allows the control of this situation. If the recipientsharedkey wasn't obtained at the first call, the client will try to obtain it when the first message from the recipient atSign is read.

Describe alternatives you've considered

No response

Additional context

No response