Remove note about sleep.

@pcostell says not to bother.
Move copy-deferred-keys-to-request logic out of 'Connection.lookup'.

Addresses @dhermes comment:
https://github.com/GoogleCloudPlatform/gcloud-python/pull/429/files#r21997643
Squash checks of protobuf call headers into helper method.

Addresses @dhermes comments:
https://github.com/GoogleCloudPlatform/gcloud-python/pull/429/files#r21998236
https://github.com/GoogleCloudPlatform/gcloud-python/pull/429/files#r21998714