SCORU/Node: GC for stores

Context

This MR implements an asynchronous garbage collection function in the stores modules (both for stores which consist of a simple index, and for stores which consist of a data file and an indexed, i.e. indexed files).

The GC is implemented following the same philosophy used in the L1 node. Each store is splitted in two (or more) internal stores: one "fresh" for new elements added to the store, and one (or multiple) "stale" for elements that were added before and that could be GCed.

flowchart LR
    stale1 --- fresh

    classDef default stroke:#333,stroke-width:2 
    style stale1 fill:#aabb88,color:white
    style fresh fill:#22ff77

When a GC starts, a new fresh store is created and the previous fresh store becomes stale. This allows to continue to add elements to the store while the GC executes in the background. A temporary store is also created to perform the actual GC.

flowchart LR
    stale1 --- stale2 --- new_fresh
    tmp

    classDef default stroke:#333,stroke-width:2 
    style stale1 fill:#aabb88,color:white
    style new_fresh fill:#bbccaa,color:white
    style stale2 fill:#22ff77
    style new_fresh fill:#ff2277
    style tmp fill:#ccccff

At the end of the GC, the temporary store becomes the new stale and the new fresh store remains.

flowchart LR
    stale1 --- new_fresh

    classDef default stroke:#333,stroke-width:2 
    style new_fresh fill:#ff2277,color:white
    style stale1 fill:#ccccff

Manually testing the MR

dune exec src/lib_layer2_store/test/main.exe -- --file test_indexed_store.ml