0.21.2 patches

Fix: ZBClient.close() and ZBWorker.close() now return an awaitable Promise that guarantees the underlying gRPC channel is closed. It takes at least two seconds after jobs are drained to close the gRPC connection. When the close promise resolves, the gRPC channel is closed. Note that ZBClient.close() closes all workers created from that client.

Feature: Log messages now include a context property with the stack frame that generated the log message.

Fix: Workers would stall for 180 seconds if they received a 504: Gateway Unavailable error on the HTTP2 transport. This was incorrectly treated as a gRPC channel failure. The code now checks the state of the gRPC channel when a transport error is thrown, rather than assuming it has failed. Fixes #96.