Use generic types instead of trait objects in tx pool

[hashmap]

Tx pool takes some parameters as trait objects. It's not an idiomatic Rust code, in this particular case we should use generic types. Trait object makes sense when we accept in runtime different concrete types which implement the trait as a value of the same field. It's not the case here. Trait objects come with a price - instead of method dispatch in compile time we have to accept runtime dispatch. My guess we did it to not clutter the code with type parameters, which is understandable but still suboptimal.

Basically the change is to replace

pub struct TransactionPool {
        pub blockchain: Arc<dyn BlockChain>,
	pub verifier_cache: Arc<RwLock<dyn VerifierCache>>,
	pub adapter: Arc<dyn PoolAdapter>,
        ...
}

with

pub struct TransactionPool<B, P, V>
where
	B: BlockChain,
	P: PoolAdapter,
	V: VerifierCache,
{
     pub blockchain: Arc<B>,
     pub verifier_cache: Arc<RwLock<V>>,
     pub adapter: Arc<P>,
...
}

The rest is just passing type parameters to Pool and TransactionPool

[quentinlesceller]

👍 Looks good to me.