Implement the namespace parameter for GetTask

[markstory]

In small sentry deployments (self-hosted, local-dev, small single-tenant) we will only have a single taskbroker topic. To enable symbolicator to operate as a taskworker, we'll need to provide a way for symbolicator to fetch tasks that it can run.

• Implement the namespace parameter of GetTask in taskbroker. Clients should be able to use this parameter to fetch tasks from a single namespace only.