sipaccountbase: add getLastMessages()

Because a client can receives messages before a client is ready to
receive it and because the daemon does not store any message, we
should have, like calls or file transfers, have a method to get
these messages.

So, this patch introduces a new method which gives the ability to
retrieve messages since the daemon has been launched. To do that,
when the daemon will receives a new message, it will store this
message into a queue (limited to 1000 messages) and save the
related timestamp. When the client is ready it can call getLastMessages
with its last known timestamp. This method will return all messages
received after this timestamp.

Note: the perfect way to do that is to store messages directly in
the daemon not clients. We also need some synchronization processes.

Change-Id: Iceb1654088a843f9be5b4a47bcc23201e9b38c01