mempool: modify maintain_transaction_pool to prune txs

[tcoratger]

Should close #1478

• We maintain a hashmap of transactions that were at one time in the pool with the timestamp of the first time we found them.
• We empty this hashmap with the mined transactions that are progressively included in the last block and that are pruned correctly.
• In addition, we also prune transactions that are still in the mempool at the moment we look and that have been in it for more than 5 minutes.
• A unit test is here to validate the proper behaviour.

cc @greged93 @ClementWalter @eugypalu