This changes when we write the drpcheader. Rather than making it its own
write to the connection, it now prepends the drpc header to the first
write on the connection (typically the tls handshake). This results in
one less packet being sent at the beginning of each drpc connection.
For an operation like uploading a file from uplink, this results in many
packets being dropped: one when communicating with the satellite, and
one for each communication with the storage nodes.
Change-Id: I7644b46e90ffa7acea73ac56831396307352ed7a
0008aebf80