lib/nfs/Connection: broadcast error before closing connection

During the NfsLease::OnNfsConnectionFailed() call, the old (defunct)
nfs_context may be used to close file handles.  Such code does not yet
exist, but will be added soon to fix other bugs.
lib/nfs/Manager: defer NfsConnection destruction

Avoids a crash that occurs when NfsConnection::OnSocketReady()
dereferences itself before returning.
lib/nfs/FileReader: update "state" in OnNfsError()

Clean up the "state" to indicate that there is no longer any
asynchronous operation.  Fixes another NFS-related crash due to
cleanup of a non-existing asynchronous operation.