[2 / 5] Make approval-distribution logic runnable on a separate thread (

#4845)

This is part of the work to further optimize the approval subsystems, if
you want to understand the full context start with reading
#4849 (comment),

# Description

This PR contain changes to make possible the run of multiple instances
of approval-distribution, so that we can parallelise the work. This does
not contain any functional changes it just decouples the subsystem from
the subsystem Context and introduces more specific trait dependencies
for each function instead of all of them requiring a context.

It does not have any dependency of the follow PRs, so it can be merged
independently of them.

---------

Signed-off-by: Alexandru Gheorghe <alexandru.gheorghe@parity.io>