TP.Bundler: don't send dest and src addresses for individual messages

When sending bundled messages to a given destination, e.g. P, if we have 10 messages with dest=P and src=S, then we marshall the message list as follows:

length=10
M1
M2
...
M10

The dest and src fields for each message is marshalled as well, and since an Address (UUID) has ca 20 bytes, the cost for dest and src address is ca 40 bytes per message. If we change the marshalling to this scheme:

length=10
dest=P
src=S
M1 (dest=null, src=null)
M2 (dest=null, src=null)
...
M10 (dest=null, src=null)

, we save (N-1) * 40 bytes == 360 bytes for the 10 messages !