Reserve UTXOs during transaction building.

[vladimirfomene]

This PR solves #186.It add functionality to mark UTXOs as selected/reserved once they have been selected by a transaction during the transaction building process. Therefore the wallet cannot choose this reserved UTXOs for subsequent transaction building.

[codecov-commenter]

Codecov Report

Merging #200 (d0f179e) into master (0fb4e1b) will increase coverage by 2.15%. The diff coverage is 0.00%.

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@@            Coverage Diff             @@
##           master     #200      +/-   ##
==========================================
+ Coverage   56.74%   58.89%   +2.15%     
==========================================
  Files          10        9       -1     
  Lines         252      236      -16     
==========================================
- Hits          143      139       -4     
+ Misses        109       97      -12     

Impacted Files	Coverage Δ
bdk_chain/src/keychain.rs	51.56% <0.00%> (-5.34%)	:arrow_down:
bdk_chain/src/file_store.rs

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[LLFourn]

Just a quick comment these methods and state should be on KeychainTracker not on the indexes. The indexes are monotone and have no way of knowing if something is spent or not in the canonical chain. Another reason they should be there is they affect the output of Balance.

They should also not have a txid argument as mentioned by @evanlinjin. You should be able to reserve a utxo without knowing what txid it's reserved for.

[vladimirfomene]

@LLFourn @evanlinjin updated as requested. Thanks for the quick review!

[evanlinjin]

Another reason they should be there is they affect the output of Balance.

Ahh now I understand why this feature is needed!

[evanlinjin]

@vladimirfomene Are the balances affected when we mark an outpoint? Maybe we can write a test for it.

[vladimirfomene]

@evanlinjin the balances are not yet affected. Let me think of a strategy

[vladimirfomene]

@evanlinjin @LLFourn written test showing how balance are affected when UTXOs are marked as selected. I have added a field for total selected amount to Balance. The idea is to create some form of visibility from a user's perspective on where the UTXOs are if for some reason they request the balance after creating a transaction and not broadcasting it.