fn: locked mutex while blocked on I/O considered harmful (#935)

* fn: mutex while waiting I/O considered harmful

*) Removed hold mutex while wait I/O cases these
included possible disk I/O and network I/O.

*) Error/Context Close/Shutdown semantics changed since
the context timeout and comments were misleading. Close
always waits for pending gRPC session to complete.
Context usage here was merely 'wait up to x secs to
report an error' which only logs the error anyway.
Instead, the runner can log the error. And context
still can be passed around perhaps for future opencensus
instrumentation.

api/runnerpool/runner_pool.go

@@ -18,7 +18,7 @@ type Placer interface {
 // RunnerPool is the abstraction for getting an ordered list of runners to try for a call
 type RunnerPool interface {
 	Runners(call RunnerCall) ([]Runner, error)
-	Shutdown(context.Context) error
+	Shutdown(ctx context.Context) error
 }
 
 // PKIData encapsulates TLS certificate data