LastMessage in chat was encoded in bytes so that we don't have to
encoded/decode everytime we save to db or pass the client.
An issue with emoji surfaced a problem with this approach.
Chat.LastClockValue represent the last clock value of any type of
message exchanged in a chat (emoji,group membership updates, contact
updates).
So when receving a new message, we should update LastMessage if the
clock of the LastMessage is lower than the received message, and we
should not only check LastClockValue, otherwise the message might be
discarded although it is the most recent.
This commit fixes the issue by keeping LastMessage as an object and
comparing LastMessage.Clock instead of LastClockValue
Andrea Maria Piana