Introduce Runtime object allowng to detect outer runtime context

[tnull]

Closes #491

Instead of holding an Arc<RwLock<Option<Arc<tokio::runtime::Runtime>>>
and dealing with stuff like tokio::task::block_in_place at all
callsites, we introduce a Runtime object that takes care of the state
transitions, and allows to detect and reuse an outer runtime context.

We also adjust the with_runtime API to take a tokio::runtime::Handle
rather than an Arc<Runtime>.

We then also go ahead an reuse said Runtime object for VssStore.

(cc @andrei-21)