Using wrangler.ctx in a lot more places.
Add unit tests for etcdtopo.
Add topo tests for deleting keyspace, shard, etc.
Merge branch 'master' into replication
Merge pull request #245 from youtube/replication

Using wrangler.ctx in a lot more places.
Another pass at context clean up.
Merge pull request #249 from youtube/replication

Another pass at context clean up.
Replacing zkocc with vtgate in this test.
Merge pull request #250 from youtube/replication

Replacing zkocc with vtgate in this test.
Allow trace plugins to be registered from outside the package.

Fixes #248
update doc to reflect changes in vtctl
Merge pull request #252 from yaoshengzhe/fix_doc_vtctl

update doc to reflect zk path changes in vtctl
Reject lock attempt on nonexistent keyspace/shard.
Merge pull request #251 from youtube/trace

Allow trace plugins to be registered from outside the package.
Removing zkocc from zkocc_test.py. Now it's only a topology test.

(keeping it with a weird name as it'll go away once vtgate v2 is
the only supported way to make queries).
Removing more zkocc tests.
Removing zkocc and related files.
Merge branch 'master' into replication
Fix panic and hang in etcdtopo lock.

Found during integration tests:
- Panic was because nil *Response is returned on error.
  Updated fake etcd client to reflect this.
- Hang was because released locks were setting empty contents
  instead of the new magic value.
Don't check ctx.Err() unless ctx.Done() was closed.

The zktopo lock tests have been flaky, with errors saying "interrupted".
This was because there's a race condition where ZK can time out before
the ctx.Done() channel has been closed, so ctx.Err() returns nil and we
return the wrong topo error (interrupted instead of timeout).