ice: refactor using pimpl idiom

Continue the compilation firewall work.
Also update code to use C++14 features.

Rationale: prepare the change to a new transport
abstraction over the project.

Change-Id: I4d387bea8447f5dc251a6bd64ecb63b234bd1cdf
Reviewed-by: Olivier Soldano <olivier.soldano@savoirfairelinux.com>