Remove code duplication - PoET harness and client

[fasmat]

Both spacemeshos/poet (see harness.go) and spacemeshos/go-spacemesh (see poet_test_harness.go) contain a harness that can be used for (integration) testing against poet.

It would be beneficial if we would consolidate these two into the poet/integration package.

Additionally the code for the HTTPPoetClient can be found in spacemeshos/go-spacemesh (see poet.go) and might make sense to move to the spacemeshos/poet repository as well.

Having the client in the poet repository would make maintenance of it easier, since it would force us (assuming we move the tests along with it) to update the client whenever we update the server.

[poszu]

The harness.go (whole integration/ dir) has already been removed from this repository some time ago.

Regarding moving the client to this repository - I'm not sure. Different use cases have different needs and it will be impossible to create a client fitting everybody.