This patch introduces a new class, SuffixStrategy and two sub-classes, SuffixContent and SuffixManifest which allow to independently assign suffixes to contents and manifests respectively. The produce() function in socket_producer.cc has also been changed to use them and has been slightly refactored (remove hard coded values, comments...).

Given a strategy and an offset (and optionally the capacity of a manifest), these classes automatically compute the correct next suffixes for both type of data (manifest or content). This removes the burden of having to manage suffixes, for instance when producing or when retrieving content, and could be expanded to add more strategy in the future.

Currently the only existing strategy is "INCREMENTAL": manifests with capacity N have a suffix multiple of N+1: 0, N+1, 2(N+1) etc. Contents have a suffix incremented by 1 except when it conflicts with a manifest: 1, 2, ..., N, N+2, N+3, ..., 2N+1, 2N+3...