grpc-js: Improve event sequencing when handling connection drops

[murgatroid99]

This changes a few things to try to report connection drops with the UNAVAILABLE status more consistently:

• The transport now passes the connection drop event along to the call even if a GOAWAY has already been handled. This is important in the server-side graceful connection shutdown case, in which the server always first sends a GOAWAY and then drops the connection seconds later.
• The transport now passes the connection drop event along immediately instead of waiting with a setImmediate, to ensure that the call sees the event as quickly as possible.
• The call now delays handling the event with a setImmediate to mimic the old sequencing in most cases, but first flags that a connection drop event was seen. User reports indicate that Node (or nghttp2) probably synthesizes a RST_STREAM with the code CANCEL when a connection drops, so if the connection drop flag is set we treat that code as UNAVAILABLE instead of CANCELLED.