Make ValidationPool accepts execution mode to run custom command or in process validation (#1622)

Squashed commit of the following:

commit 3eec1ef0
Author: Cecile Tonglet <[email protected]>
Date:   Wed Sep 2 11:43:47 2020 +0200

    CLEANUP

    Forked at: bf7ccb84
    Parent branch: origin/master

commit 50ba4847
Author: Cecile Tonglet <[email protected]>
Date:   Wed Sep 2 11:42:07 2020 +0200

    Add test

commit 33ced08b
Author: Cecile Tonglet <[email protected]>
Date:   Wed Sep 2 11:21:57 2020 +0200

    Run validation inside the same process

commit 4e113ba0
Author: Cecile Tonglet <[email protected]>
Date:   Wed Sep 2 10:25:06 2020 +0200

    Adapt code to review suggestions

commit f46e7a71
Merge: 6301515b e2ebf25b
Author: Cecile Tonglet <[email protected]>
Date:   Wed Sep 2 10:03:04 2020 +0200

    Merge commit e2ebf25b (no conflict)

    Parent branch: origin/master
    Forked at: bf7ccb84

commit 6301515b
Merge: 7f573736 01ffc66b
Author: Cecile Tonglet <[email protected]>
Date:   Tue Aug 25 01:09:27 2020 +0200

    Merge commit 01ffc66b (no conflict)

    Parent branch: origin/master
    Forked at: bf7ccb84

commit 7f573736
Merge: 1fd29876 00202d37
Author: Cecile Tonglet <[email protected]>
Date:   Tue Aug 25 01:09:22 2020 +0200

    Merge commit 00202d37 (conflicts)

    Parent branch: origin/master
    Forked at: bf7ccb84

commit 1fd29876
Merge: 41cb5b5f 62cf5b97
Author: Cecile Tonglet <[email protected]>
Date:   Tue Aug 25 01:05:23 2020 +0200

    Merge commit 62cf5b97 (no conflict)

    Parent branch: origin/master
    Forked at: bf7ccb84

commit 41cb5b5f


Author: Cecile Tonglet <[email protected]>
Date:   Tue Aug 25 01:03:31 2020 +0200

    Update service/src/lib.rs

Co-authored-by: Bastian Köcher <[email protected]>

commit 8a5aa081
Author: Cecile Tonglet <[email protected]>
Date:   Fri Aug 21 19:06:36 2020 +0200

    WIP

    Forked at: bf7ccb84
    Parent branch: origin/master

commit b7a97df8
Author: Cecile Tonglet <[email protected]>
Date:   Fri Aug 21 15:56:21 2020 +0200

    Fix test

commit e28eed75
Author: Cecile Tonglet <[email protected]>
Date:   Fri Aug 21 15:37:13 2020 +0200

    Update validation/src/validation_service/mod.rs

commit 8c880f71
Author: Cecile Tonglet <[email protected]>
Date:   Fri Aug 21 15:02:45 2020 +0200

    Propagate test mode all the way down to ValidationPool

commit d32b42a2
Author: Cecile Tonglet <[email protected]>
Date:   Fri Aug 21 15:27:56 2020 +0200

    Initial commit

    Forked at: bf7ccb84
    Parent branch: origin/master