refactor(chain)! SyncRequest ergonomics improvements

[LLFourn]

This is an attempt to improve the ergonomics of sync request especially with regards to inspecting the progress of the sync.
Now there is a single inspect method which takes a closure which gets richer information than the previous ones. It takes:

(SyncItem<'a, I>, usize, usize)

where the usizes are the index of the current item and the number of items remaining respectively.
SyncItem is defined as:

/// An item reported to [`inspect`]
///
/// [`inspect`]: SyncRequest::inspect
pub enum SyncItem<'a, I> {
    /// Script pubkey sycn item
    Spk(I, &'a Script),
    /// Transaction id sync item
    Txid(Txid),
    /// Transaction outpoint sync item
    OutPoint(OutPoint),
}

Note carefully the type parameter I inside which is just to help log out the index (or whatever metadata you want) along with the spk.

This is not ready for review but I thought @evanlinjin might be interested in finishing this work.

[evanlinjin]

Replaced by #1478