ledger-db restructuring

[Ben-PH]

• document all added functions
• try in sandbox /simulation/labnet
• unit tests on the added/changed features
  • make tests compile
  • make tests pass
• add logs allowing easy debugging in case the changes caused problems
• if the API has changed, update the API specification

[Ben-PH]

I've been looking into making the results of the getters return a byte-slice instead of a vec as well. The issue is that the library uses the DBPinnableSlice struct to manage the lifetime of the data-pinning. When it goes out of scope, a rocksdb defined ffi function is used to unpin (free up) the data. Fundamentally, the question seems to be this:

How to make the v-table containing the drop method available outside of massa-ledger-worker library, without making our other libraries also dependent on rocksdb (which leads to longer compile times. like, really long)

[Eitu33]

We usually employ "refactor" when dealing with major behavioral changes of a module, I changed the title to restructuring to avoid misleading anyone. Also I'm not a big fan of the ledger_db subdir, I don't feel there is a need for that as the module is entirely related to the the ledger_db file anyway, feels weird to have a file and a directory with the same name too, I'd rather we keep everything at the root. Otherwise lgtm.

[Ben-PH]

I'll revert file separation, then merge on monday.

I'll also raise a discussion in github about that (I personally prefer not to have mod.rs files, but I'll raise it in the discussion)