* Move out node-pool manager and replace it with RunnerPool extension
* adds extension points for runner pools in load-balanced mode
* adds error to return values in RunnerPool and Runner interfaces
* Implements runner pool contract with context-aware shutdown
* fixes issue with range
* fixes tests to use runner abstraction
* adds empty test file as a workaround for build requiring go source files in top-level package
* removes flappy timeout test
* update docs to reflect runner pool setup
* refactors system tests to use runner abstraction
* removes poolmanager
* moves runner interfaces from models to api/runnerpool package
* Adds a second runner to pool docs example
* explicitly check for request spillover to second runner in test
* moves runner pool package name for system tests
* renames runner pool pointer variable for consistency
* pass model json to runner
* automatically cast to http.ResponseWriter in load-balanced call case
* allow overriding of server RunnerPool via a programmatic ServerOption
* fixes return type of ResponseWriter in test
* move Placer interface to runnerpool package
* moves hash-based placer out of open source project
* removes siphash from Gopkg.lock
