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 #156 from youtube/resharding

Better parameter parsing for strategy.
Merge pull request #154 from youtube/tabletmanager-context

Add tracing in rpcplus and other places downstream from tabletmanager.
v1 of writeMastersOnly strategy for split_clone workers
Passing tests for v1 of writeMastersOnly strategy
Break out TestSplitClone into multiple tests
Modify sharding integration tests for vtworker to use writeMastersOnly strategy
Make vtworker find master tablets more efficiently when using writeMastersOnly strategy
Make writeMastersOnly into a proper parsed strategy
Merge pull request #153 from youtube/aaijazi_write_masters_only_strategy

writeMastersOnly strategy for split_clone workers