Remove dup tx check from mempool

[edsko]

This would first require to fix the tx request client though.

Related but orthogonal: #1743

[edsko]

This should be easy to do now, we can just check the cache from the mempool. It would not reintroduce the "recent txs" cache.

[edsko]

From slack: the tx submission client:
it should check the mempool whether or not to request a tx at all
and then, I guess, again when it actually gets it
since it might have been inserted in the meantime
that last check is going to be very annoying actually now that I think about it
will be hard to avoid a race condition

[mrBliss]

From slack: the tx submission client:
it should check the mempool whether or not to request a tx at all
and then, I guess, again when it actually gets it
since it might have been inserted in the meantime
that last check is going to be very annoying actually now that I think about it
will be hard to avoid a race condition

To expand on the second check: the second check avoids adding a transaction already in the Mempool, which will cause the transaction to be rejected. This will pop up in the trace and might confuse users.

IMO, this is not a big deal, we can just do the first check so we don't request a transaction already in the Mempool. If we then request and receive the same transaction from two nodes, one of them will be rejected, so be it. #1743 will help with that, except that the wallet could still add the same transaction between the time we request it from another node and add it to the Mempool.