toxcore removes message receipts and filetransfers from memory, when client has short network outage

[iphydf]

Based on #548.

when on a mobile phone you can be offline rather often (elevator, parking garage, bad cell service, tunnel ...) then you go offline and online again.
but now you will not receive message read notifications and you lose all FT numbers.
i don't think this is good

https://github.com/TokTok/c-toxcore/blob/master/toxcore/Messenger.c#L930

break_files(m, friendnumber);
clear_receipts(m, friendnumber);

what if we removed these 2 lines?
(or maybe add a timespan, only when offline for more than 5 minutes lose these values.)
keep the data in memory, like you were online the whole time.

comments?