storage: make ReclockFollower shareable

[guswynn]

As part of BIR, the source reader operator will need access to the reclock trace to "invert" a resumption_frontier into a upstream commit frontier. This pr makes the ReclockFollower share-able, within a single thread (worker), by using Rc<RefCell>. This should have a relatively low cost. This pr does NOT implement the inversion logic.

Notes:

• I chose the simplest route, which is to protect ALL data within the RefCell
• The reclock nested-iterator had to changed to a single for_each. @petrosagg came up with a way to avoid this with Yoke, but that seems needlessly complex for our usecae.
• Note that borrow_mut() users keep their &mut signatures. This can ensure that we reduce the chance of writing code that will hit a borrow error, while keep the api clean (ReclockFollower: Clone)

Motivation

• This PR adds a known-desirable feature.
  part of BIR

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered.
• This PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way) and therefore is tagged with a T-protobuf label.
• This PR includes the following user-facing behavior changes:

[aljoscha]

I think the code changes look good. I don't much like the pattern of things that are Clone but are only doing a shallow copy and are then sharing state. I don't know how common this is in the rust world so I'll defer to Gus and @petrosagg. We might have a shallow_clone() method or sth like it but, again, I don't know how idiomatic that would be.

[guswynn]

@aljoscha I kindof agree, i can make a fn share(&self) -> Self instead of implementing Clone!

[aljoscha]

@aljoscha I kindof agree, i can make a fn share(&self) -> Self instead of implementing Clone!

I like that!