Add adapter interface for tracing libraries.
Add tracing in rpcplus and other places downstream from tabletmanager.

The goal is to start getting useful traces out of tabletmanager.
To start, we trace outgoing RPC calls on the client side.
Certain high-level TopoServer operations are also traced at this point.

To facilitate tracing, I've begun plumbing Context through tabletmanager
and all its users. At this time, it is simply a few regexes adding the
parameter so it can be used by tracing; no timeouts or cancellation are
using Context yet. Those use cases will require more careful
consideration.

Other than tabletmanager, most callers use context.TODO() since they
aren't plumbed yet.
Merge pull request #154 from youtube/tabletmanager-context

Add tracing in rpcplus and other places downstream from tabletmanager.