grpc-js: Fix a couple of things that came up while investigating a memory leak

[murgatroid99]

The first fix, in client.ts is to null the callerStackError variable before the onReceiveStatus callback returns. The Error object retains information about the stack trace where it was created, including context objects. In the specific case where a new request is initiated while handling the result of a previous request, the new request's callerStackError would retain a stack context that includes the previous callerStackError. If done in a ping-pong type loop, this retention could be recursive.

The second fix is to ensure that non-OK statuses caused by picker errors (especially if the channel is not connected) are emitted asynchronously. If emitted synchronously, if calls are retried unconditionally and synchronously, the process could make no progress towards reconnecting, resulting in an infinite loop.