The new WebService get_documents() is more efficient in terms of
server resources consumption since it doesn't force the WS engine
to calculate the full result set length, which had little use anyway
in Rigo.
This commit switches entropy.client.services' DocumentList to and
reverse dependencies to use the new WS API. The older WS API will be
kept alive for a while (6 months, roughly).
Introduce Entropy conditional dependencies supoort if
ETP_PORTAGE_CONDITIONAL_DEPS_ENABLE is set in the environment.
This feature is disabled by default because it braks backward
compatibility and older Entropy Clients are broken wrt this.
