Work around a DRBD sync speed race condition

This is modified forward-port of commit 1544 on the 1.2 branch:

  When DRBD is doing its dance to establish a connection with its
  peer, it also sends the synchronization speed over the wire. In
  some cases setting the sync speed only after setting up both
  sides can race with DRBD connecting, hence we set it here before
  telling DRBD anything about its peer.

  Reviewed-by: iustinp

The modification we make is that we split SetSyncSpeed in two so that we
don't need to modify our minor temporarily, and the fact that we call
this function from within _AssembleNet (right before enabling network),
instead of Assemble()/Attach().

Original-Author: imsnah