Sends all messages currently in forward_table to the new coordinator (changing the dest field).
This needs to be done, so the underlying reliable unicast protocol (e.g. UNICAST) adds these messages
to its retransmission mechanism
Note that we need to resend the messages in order of their seqnos ! We also need to prevent other message
from being inserted until we're done, that's why there's synchronization.
Access to the forward_table doesn't need to be synchronized as there won't be any insertions during flushing
(all down-threads are blocked)