limits dependencies for plugins

Plugins need to be able to (de)serialize inputs and outputs, but allowing them to access `Llio` opens up `Std` and explodes the number of dependencies. These dependencies make _plugins_ break if they are not compiled with the exact (md5 wise) same set of library versions as the arakoon binary.

The patch introduces an extra interface allowing them to not be dependent on kitchen sinks like `Lwt` or `Std`. 
As an example: 
```
ocamlobjinfo _build/plugin_find.cmo
File _build/plugin_find.cmo
Unit name: Plugin_find
Interfaces imported:
	ad06f04cfca6d404d1de76c3dc67324a	Int32
	d012329cc712e91d0f10a5eef2303d18	Printf
	db7f34081ef8fcaf499f19523d0736c6	String
	9b043210ba766ec98c0c64e3dfba3013	Plugin_find
	e2378c62e62b3516b3b9da1a984c728d	Plugin_helper
	af3ef6fba94cdb4eba31e98b4e341dab	Buffer
	024edc3512403b725052aec8e41ed971	Hashtbl
	b0adfa4175f86e4394859886c1a374bb	Obj
	36b5bc8227dc9914c6d9fd9bdcfadb45	Pervasives
	f11d7ddcffad09397202a49bd9bb4283	Map
	6c35bd33088907f850bc27cd9072ffb5	Registry
Uses unsafe features: no
Force link: no
```

While previously it would fe also depend on `Test_utils`, `Quickcheck` and some 40 others.
Maybe mention this is a hint for future implementors (and when/why they would want to do this), otherwise somebody might come and clean it up