Upgraded concurrency framework to support panic=unwind mode

[pompon0]

We are going to use era-consensus code in zksync-era, which doesn't have panic=abort enabled for the time being. This PR adjust the concurrency framework so that it behaves in the expected way in the presence of panics:

• scope::run/run_blocking will wait for all tasks to either complete OR panic
• scope's context gets cancelled immediately if any of the tasks returns an error OR panics
• scope::run/run_blocking will panic if any of the scope's tasks panicked (but only after all tasks complete or panic). This also means that even if task A returns an error before task B panics, the whole scope will panic anyway (A's error will be ignored).
• the root task of the scope::run/run_blocking call will NOT be executed in the same tokio task as the run call any more (so that a panic in the root task won't prevent scope call from awaiting for completion of all tasks).

[slowli]

I think we've discussed this previously, but it could be worth bringing up another time. If the context is canceled, wouldn't it make sense to immediately return a cancellation error from wait_blocking, even if the provided closure runs to completion (because we cannot stop it)?

[pompon0]

I haven't changed the semantics - scope waits for all tasks to finish no matter if they ignore the ctx cancellation or not.

[pompon0]

afaict, our use cases for this function are about running a blocking function in non-blocking context rather than allowing things to run after context cancellation. In general it is a bug to spawn a blocking context-unaware method from within a scope (it is against structured concurrency principle), so I don't want to facilitate such an antipattern (with the current semantics a context-unaware function should cause a deadlock which will propagate up the stack and should be relatively easy to notice). In case of rocksdb access it is ok-ish, because it is expected to finish in a bounded amount of time (in fact all filesystem accesses in unix don't have a proper notion of cancellability).