Double borrow in `observe_after_transaction`

[Waidhoferj]

Key YDoc methods are inaccessible inside observe_after_transaction callbacks due to a double borrow. For example

def test_borrow_issue():
    d = Y.YDoc()
    m = d.get_map("foo")
    update: bytes = None

    def put_updates(ydoc: Y.YDoc, event) -> None:
        nonlocal update
        update = Y.encode_state_as_update(ydoc, event.before_state) #borrowed again here FAILS

    d.observe_after_transaction(lambda e: put_updates(d, e)) # Borrowed here

    with d.begin_transaction() as txn:
        m.set(txn, "hi", "there")

[Waidhoferj]

So here's the issue: AfterTransactionEvents are fired before the transaction is freed, so that the developer has access to transaction-related attributes in the callback. The transaction holds a mutable reference to its associated YDoc so it can update the data inside. When you call encode_state_as_update, the function uses the provided YDoc to create a second new transaction. In doing so, it borrows YDoc again mutably, which isn't permitted because there cannot be 2 mutable references to the same element according to Rust ownership rules.

In #73, I added a way to do the same thing from the AfterTransactionEvent passed to the callback. Its generally a better idea to use the event if possible. @davidbrochart lmk if you need any additional functionality from the AfterTransactionEvent.

[davidbrochart]

For now this is all I need, thanks!