[PERF] Transactions in TransactionSession should have a TTL

[veeso]

Description

Currently if a transaction is not closed by the user, it lives forever.

Of course not any user can create a transaction, so the peer creating the transaction should decently be able to handle transactions correctly. This means that currently, the impact should not be huge, since the caller should always either rollback or commit a transaction.

But anyway we should provide an extra argument when we create the transaction to provide a ttl, which must be an option. A None value means the caller is responsible for closing the transaction manually and we are not responsible in any way for cases where the closing of the transaction has not happened.

This also requires then a background process which with a fixed interval will close expired transactions and must be added to the proc macro to create the dbms canister.