The removeTransportAddress method of TransportClient removes the address
from the list of nodes that the client pings to sniff for nodes.
However, it does not remove it from the list of existing connected
nodes. This means removing a node is not possible, as long as that node
is still up.

This change removes the node from the connected nodes list before
triggering sampling (ie sniffing).  While the fix is simple, testing was
not because there were no existing tests for sniffing. This change also
modifies the mocks used by transport client unit tests in order to allow
mocking sniffing.