introduce IpfsNode Plugin

[MichaelMure]

This PR introduce a new kind of plugin able to directly use IpfsNode. As IpfsNode is considered internal, no guarantee is made about API stability. This is useful when a plugin needs a direct access to the internal and using CoreApi is not enough.

As introducing PluginNode make the naming a little akward, this PR also introduce a copy of PluginDaemon named PluginApi and deprecate PluginDaemon. As it is a separate commit, we can remove that easily if you don't like it.

[MichaelMure]

golint fail because PluginDaemon is now deprecated. Other CI failures seem unrelated.

[Stebalien]

(finally reviewing this)

Code LGTM I'd like to revert the rename and use:

• PluginDaemon - uses the API.
• PluginDaemonInternal - uses the raw node and makes it clear that this is an internal plugin.

I called it "PluginDaemon" instead of "PluginAPI" because it follows the lifecycle of the daemon itself.

[MichaelMure]

Updated as you wanted.

[Stebalien]

Thanks!