Higher-priority transactions can now replace lower-priority transactions
even when the internal _tx_mem_pool_ is full.

**Notes for reviewers:**
- The _tx_mem_pool_ now maintains information about transaction
priority. Although _tx_mem_pool_ itself is stateless, transaction
priority is updated after submission to the view. An alternative
approach could involve validating transactions at the `at` block, but
this is computationally expensive. To avoid additional validation
overhead, I opted to use the priority obtained from runtime during
submission to the view. This is the rationale behind introducing the
`SubmitOutcome` struct, which synchronously communicates transaction
priority from the view to the pool. This results in a very brief window
during which the transaction priority remains unknown - those
transaction are not taken into consideration while dropping takes place.
In the future, if needed, we could update transaction priorit...