devp2p: solve "DPT discovers nodes when open_slots = 0" (#1816)
devp2p: solve "DPT discovers nodes when open_slots = 0" (#1816)

* devp2p: remove OpenSlots check
in favor of queue length cap

* devp2p/rlpx: remove extraneous `peer:` label

Co-authored-by: Ryan Ghods <ea3cd978650417470535f3a4725b6b5042a6ab59@ryanio.com>

* devp2p/rlpx: add helper function
`_getOpenQueueSlots()`

Co-authored-by: Ryan Ghods <ea3cd978650417470535f3a4725b6b5042a6ab59@ryanio.com>
ethereumjs/ethereumjs-vm,ethereumjs/ethereumjs-vm,ethereumjs/ethereumjs-vm,ethereumjs/ethereumjs-vm,ethereumjs/ethereumjs-vm