Post CloudCommandProxy::SendCommandUpdate to accumulate more changes

Single message loop usually updates several properties of command.
Accumulating them we can reduce number of requests to server.

Change-Id: I57f6ea7e15192c6c1a22dafd66b3add62aee846e
Reviewed-on: https://weave-review.googlesource.com/1263
Reviewed-by: Vitaly Buka <vitalybuka@google.com>