jamiaccount: cache compressed vcard given by the client in sendTrustRequest

For now, the daemon doesn't handle the profile of the account nor
the compression. However, it does handle sendTrustRequest, which uses
the DHT to transmit requests. DHT values must be small, else it will
not be sent. So, this patch cache the compressed payload generated
by the client and removes it as soon as the confirmation is received.
This allow the daemon to retry to send request with the payload sent
originally.

Change-Id: I3fac542c6a53febdb7f1d0c516ec2d9ff5b317be