Part of #1543
Closes #1550

Description

Instead of updating a TxGraph with another TxGraph in .apply_update(), we introduce tx_graph::Update. tx_graph::Update is a simple data object. This is the first step of #1543. This also makes it slightly less expensive to create an update.

Additionally, we simplify the update logic of TxGraph by containing most of the update logic in .insert_{}-esc methods. .apply_update and .apply_changeset will call .insert_{} methods internally and is greatly simplified. Thus, we also get rid of .determine_changeset.

We change .apply_update methods of TxGraph,IndexedTxGraph and Wallet to implicitly set seen_at to the current timestamp for unconfirmed update transactions. This makes these methods only available when the "std" cargo feature is enabled. For a non-std environment, we introduce .apply_update_at (as shown below).

pub fn apply_update_at(&mut self, update: Update<A>, seen_at: Option<u64>) -> ChangeSet<A> {
    todo!()
}

During this process, I've fixed some tests that didn't make much sense.

Notes to the reviewers

There is a slight scope-creep, but they all address the update logic of TxGraph and structures that build on top.

Changelog notice

• Changed update API (.apply_update) of TxGraph to take in a simple data object (tx_graph::Update).
• Changed .apply_update methods of TxGraph, IndexedTxGraph and Wallet to implicitly set the seen_at to the current timestamp for unconfirmed update transactions (this making them depend on the "std" cargo feature).
• Add .apply_update_at which is the no-std version of .apply_update.

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature