account: add sendReadReceipt preference

This avoids to duplicate logic for all clients and fix the behavior
for swarm. setMessageDisplayed should be called everytime by the
client. Then the daemon will refresh the lastDisplayed for account
and will or not send the notification to other peers.
Note: if notifications are not sent, incoming displayed notifications
are also ignored. If we don't send we don't receive too.

Change-Id: If0f462ac4670d50a114c4a54bd90d76d83084bd9
GitLab: #599