Shell: fix possible stack overflow in validation.

Ouch! That was a subtle Lwt misuse.

With the current (very-old) validator code, when the validation of
block is waiting to the validation of its predecessor, a "pending" Lwt
thread is created. The validation of the predecessor might also wait
on its own predecessor, potentially creating a very long chain of
pending validation"... If in the process one of the block is tagged
invalid, all the pending "successors" in the chain are 'wakeuped'
immediatly and in sequence, potentially blowing the stack in the
process.

A quick fix is to add an `Lwt_unix.yield` to break the recursion.

A better fix is to not create such long chain of "pending" validations.
See merge request !59.

src/node/shell/validator.ml

@@ -474,6 +474,7 @@ module Context_db = struct
     let net_state = Distributed_db.state v.net_db in
     get_context v block.Block_header.shell.predecessor >>= function
     | Error _ as error ->
+        Lwt_unix.yield () >>= fun () ->
         set_context v hash (Error [(* TODO *)]) >>= fun () ->
         Lwt.return error
     | Ok _context ->