dnet_cmd: all members of the packed structures must be accessed via special getters()
keys: added keys array structure. It sorts provided data and allows to find key. Used by BulkRemove() operations
remove: remove callback returns command now
session: remove callback returns cmd,key,error structure. added BulkRemove() method.
Merge branch 'v2.26' of github.com:bioothod/elliptics-go into 2.26

Conflicts:
	elliptics/ctogo.go
	elliptics/key.go