Add IBlockPolicy.IsTransactionValid

[earlbread]

This adds IBlockPolicy.IsValidTransaction method which is a method to determine if a transaction is valid.

[codecov]

Codecov Report

Merging #827 into master will increase coverage by 0.06%.
The diff coverage is 99.28%.

@@            Coverage Diff             @@
##           master     #827      +/-   ##
==========================================
+ Coverage   87.38%   87.44%   +0.06%     
==========================================
  Files         243      244       +1     
  Lines       22156    22288     +132     
==========================================
+ Hits        19360    19489     +129     
  Misses       1453     1453              
- Partials     1343     1346       +3     

Impacted Files	Coverage Δ
Libplanet.Tests/Blockchain/BlockChainTest.cs	98.73% <98.03%> (-0.03%)	⬇️
Libplanet.Tests/Blockchain/NullPolicy.cs	88.88% <100.00%> (+1.38%)	⬆️
...lanet.Tests/Blockchain/Policies/BlockPolicyTest.cs	97.57% <100.00%> (+0.24%)	⬆️
Libplanet.Tests/Net/SwarmTest.cs	96.55% <100.00%> (-0.04%)	⬇️
Libplanet/Blockchain/BlockChain.cs	90.47% <100.00%> (+0.20%)	⬆️
Libplanet/Blockchain/Policies/BlockPolicy.cs	84.31% <100.00%> (+0.98%)	⬆️
Libplanet/Net/Swarm.cs	84.73% <100.00%> (+0.04%)	⬆️
Libplanet/Tx/TxViolatingBlockPolicyException.cs	100.00% <100.00%> (ø)
Libplanet/Net/Protocols/KBucket.cs	85.59% <0.00%> (-2.55%)	⬇️
... and 3 more

[moreal]

IsTransactionInvalid method just calls _isTransactionsValid and pass its return value. so how about using property?

Suggested change

	private readonly Predicate<Transaction<T>> _isTransactionValid;
	public Predicate<Transaction<T>> IsTransactionValid { get; }

[dahlia]

@moreal I believe the way you suggested does not satisfy the interface anyway, because in the IL level delegate properties and methods are merely distinct each other; the same looking syntax between delegate properties and methods in C# is just a coincidence.

[moreal]

@dahlia If I had understood correctly, it means that method bool IsTransactionValid(Transaction<T>) and delegate property Predicate<Transaction<T>> are distinct each other so it will not be achieved.

If then, it was what I really wanted to suggest to replace IBlockPolicy><T>.IsTransactionValid(Transaction<T>) with Predicate<Transaction<T>>. As code, it will be like below.

public interface IBlockPolicy<T>
    where T : IAction, new()
{
    // bool IsTransactionValid(Transaction<T> transaction);
    Predicate<Transaction<T>> IsTransactionValid { get; }
}

[dahlia]

@moreal That was how this patch looked before it's amended, but then it became like the current way in a reason: .NET interfaces say nothing about constructors, so an interface anyway cannot enforce its implementations to take a delegate. On the other hand, it's unclear what's benefit of having a predicate as a delegate property instead of a method, which is more common and natural.

[moreal]

Okay, I understood. Thanks for your good explanation.