The single validation module is split in multiple (simpler)
modules. In the process, we introduce one "validation worker" per
peer. This worker handle all the `New_head` and `New_branch`
advertised by a given peer. For so, it sends "fetching request" and
"validation request" to respectively the `Distributed_db` and and the
`Block_validator`. These two global workers are responsible of the
'fair' allocation of network and CPU ressources amongst the connected
'peers'.
- introduced `test/utils/test_lib.inc.sh` to simplify usage of
sandboxed node/client in the testsuite
- it reuses code from `./script/{node,client}_lib.inc.sh`
- use `wait_for_the_node_to_be_ready` to properly wait for the node to
be launched rather to use a fexed delay
- `test_multinode.sh` now launch 8 nodes.
