transport: added preprocessed policies

[Kulezi]

• removed wrapping of policies
• policy interface is now just Node(QueryInfo, n int), which returns the n-th node of the plan
• created policyInfo struct with Preprocess() method, meant to be called on topology refresh to allow calculating all possible query plans in advance, its passed to the policy inside QueryInfo for further use.

[mmatczuk]

Looking at the code I think we can squash the commits to one.