Implement TransactionPoolService

[konstantinabl]

Motivation
We need TransactionPoolService class, which will take care of communicating with the storage class and remove and save transactions from and into the pending transactions list. It will serve as a facade in front of the storage classes.

Description
Create a TransactionPoolService class that implements the TransactionPoolService interface.

Create the following methods:

• saveTransaction(address: string, tx: Transaction): Promise;
• onConsensusResult(): Promise
• getPendingCount(address: string): Promise;
• resetState(): Promise;

Add unit tests for each method