finagle: simplify Stack clients and servers. Unify around them.

We introduce StdStack{Client,Server} and apply them consistently
across clients and servers. This simplifies our current "new style"
protocol implementations and makes them more flexible. Users can now
customize, parameterize, and otherwise alter the entirety of the
finagle stack through a few, carefully selected primitives. (Namely,
Stack, Stack.Params, and StdStack*.) Client and servers are now
first-class values that may be manipulated through the use of these
primitives.

This makes consistent use of Stack for our various clients and
servers. Old uses in package exp are removed.

RB_ID=451549