Mempool: indicate minimal fees for an operation to be accepted

When an operation is rejected by the mempool because its fee is too low (e.g. because there is already an operation from the same manager with higher fees, or because the mempool is already full of operations with better fee or fee/gas ratio), the resulting error used to indicate what minimal fee it should have to be successfully reinjected. This indication has been lost in !6749 (merged) for a manager conflict and in !6787 (merged) for a full mempool, for technical reasons that make it harder to get than before. Nevertheless, it should be brought back because it is useful to the users.

• Full mempool !8573 (merged) !8640 (merged)
• Manager conflict !8699 (merged) !8842 (merged) !9016 (merged)