This patch adds the newly-introduced node flags to the design document,
as they currently are missing from there.

The patch also reduces the TOC depth to 3, as it was too big.

Reviewed-by: ultrotter

This patch removes the client-api.txt document (since it's obsoleted by
the documentation inside the design-2.0.rst document) and adds many
updates to the latter.

Reviewed-by: imsnah

This adds a simple diagram for the cluster architecture and also adds a
dependency on graphviz (both doc and in configure).

Reviewed-by: imsnah

This patch combines all the design documents for 2.0 except the
security one into a single document, in order to ease reading and reduce
duplication of information.

Future patches will start removing wrong pointers to old document names
and some better integration between the sections.

Reviewed-by: imsnah