Allow addblob commands to create output.

Currently all addblob commands remain silent even on fatal errors
because para_client either reads the command output, or sends its
stdin, but never both.

This patch overcomes this shortcoming. It makes para_client create
two buffer trees instead of one. The client states CL_SENT_COMMAND
and CL_RECEIVING are replaced by a single CL_EXECUTING state.

Blob data is now sent as a sideband packet.  The change breaks
compatibility with earlier 0.4.x versions again, but that's OK as
this is 0.5.0 material anyway.