Minor bug fixes

A couple of fairly minor changes to address some edge case issues

• DOn't throw error or reject promise if end() method called when there is no active sftp connection. If end() is called and there is no active connection, just turn into a noop. Better to call end() when not needed than miss calling it because it might cause an error in some scenarios.
• Don't throw exception or reject a promise when an 'unexpected' end or close event is seen. Logic for determining when a close or end event was 'unexpected' was not reliable enough. Besides, trying to identify unexpected close/end signals didn't really add any value. Now if a close or end signal occurs which was not 'expected', we will just log it. We do invalidate the sftp connection whenever we see an end/close event regardless.
• Now use nullish coalescing instead of or'iing to set the various params for retry configuration. Previously, using or'ing meant you couldn't set a param to 0. Using ?? now allows this.
• Made some minor changes to when the flags used to track event handling are reset. Previously, they were reset after temporary listeners were removed. We now do the reset after new temporary listeners are added.

None of the above changes are API breaking (for v9). Should be able to upgrade from any previous 9.x version with no issue.