This change fixes a race condition occurring when querying a client in
parallel with changing its context.

ddtrace: add option to force SpanID

To create a bound contract add mention that when nil error is passed it has no effect on trace.

…#396)

- Put finishWithError calls inside clojure functions because the deferred
function's arguments are evaluated when the defer statement is
evaluated. More at https://golang.org/doc/effective_go.html#defer
Before this commit, `finishWithError` deferred with nil error and
any other changes of the initial value were not added to span. This bug
affects only grpc streaming.
- Clean tagCode in case that error was disregarded.
- Add tests for checking if span contains error.

This change vendors mongo v0.2.0 inside the integration to protect
against the continuous breaking changes occurring in the library.

This change adds the `_dd.origin` tag to local-level root spans that have a remote parent which propagated the `x-datadog-origin` header. Currently it is mainly used by the Synthetics product.