Implement network related models

[umamaistempo]

This issue relates to implementing basic models that will be used to properly
implement basic network functionality

Information can be found on the wiki page about internet connections

A network connection is comprised by a Tunnel that contains Links and
Connections

Tunnel

Represent a certain medium to connect from the gateway server to the
destination server. A tunnel contains several Links. Connections use the
Tunnel.

The Tunnel model is composed by:

• Unique ID
• network_id
• gateway_id
• destination_id
• hash*

* Hash is a precomputed value generated by applying a certain hash function to
all links onto a Tunnel. This is done so a user can have multiple Tunnels to a
certain destination server and allow the backend to lazily receive the action as
a collection of nodes (and thus reuse a Tunnel)

Link

A link is the edge from a certain node used in the Tunnel to another. A Tunnel
is composed by 1 or more links and those links (and their order) affect the
side-effects of a certain network action.

On Tunnel that emanates from Node onto Node through the path
A -> B -> C -> D we have the links A -> B, B -> C and C -> D.

The link model is composed by

• tunnel_id
• source_id
• destination_id
• sequence*

* Sequence id is a precalculated integer to order the edges. This could be
avoided by using a linked list approach on the database side, but it is faster
to prototype this way

Connection

A connection represents an action that is happening through a Tunnel and that
was started by the gateway and targets the destination.

That is, if a certain user is trying to crack into a certain destination
computer through certain bounce nodes, it would start a Tunnel with those nodes
and start a "crack" connection. After the crack is complete and the user gets
into the target server, the "crack" connection finishes and a "ssh" connection
starts. If the user decides to download a file, then they have simultaneously
a "ssh" connection (because they are logged into the target system) and a "ftp"
connection (because they are transfering files to/from the target system)

The connection model is composed by

• Unique ID*
• tunnel_id
• Connection Type

* This unique id is used to allow a certain tunnel to have more than one
connection of a certain kind (ie: download a dozen of files in parallel) and
also to provide visual cue to players that two simillar connections of the same
kind are different

ConnectionType

Naturally since a connection has a type and the amount of possible types is
limited, it is better to introduce a lookup table to provide referential
integrity

About tunnel implementation

On the Tunnel's model i see the following functions:

create(Network.t, gateway :: Server.id, destination :: Server.id, bounces :: [node, ...]) :: %Tunnel{links: [Link.t, ...]}

On this case, the links builder inside tunnels model would make a set of input
nodes (and fail if any is repeated) and also ensure that the first node is the
gateway and the last node is the destination