MDS-55: Implementation of multipart
MDS-55: client can choose couple by couple_id arg

couple_id is a query_list argument means a minimum group from couple's
groups. Proxy will use couple given from mastermind by couple_id
instead of weigthed couple if couple_id is passed.
If the couple_id isn't a minimum group from couple, response will be 400
Bad Request.
If there are no enough space in couple given by couple_id, response will
be 507 Insufficient Storage.
Fixed buffer truncating in upload_buffer helper
Merge commit '0e016d8bd29b3f2fe9eaed28ce737b456b86c9d8'

Conflicts:
	src/proxy.hpp
	src/utils.hpp
Version up to 0.64