ipdb: fix `LinkedSet()` cascade updates
ipdb: fix `LinkedSet()` cascade updates

Ban updated keys from further cascade updates. That is,
when you remove a port from a transaction, it can not
occasionally return, being propagated by the third side.

Bug-Url: https://github.com/svinota/pyroute2/issues/45

roolebo/pyroute2,little-dude/pyroute2,little-dude/pyroute2,tomislacker/python-iproute2,roolebo/pyroute2,tomislacker/python-iproute2,mtiny/pyroute2,drzaeus77/pyroute2,nazarewk/pyroute2,simudream/pyroute2,drzaeus77/pyroute2,nazarewk/pyroute2,little-dude/pyroute2,vodik/pyroute2,vodik/pyroute2,mtiny/pyroute2,nazarewk/pyroute2,simudream/pyroute2