key/usercache: move deferred dialing fields into their own struct
key/usercache: move deferred dialing fields into their own struct

This silences a 'go vet' error about copying locks
and makes the code a bit more modular.

Change-Id: I6b31f53d57bedcff83e6354296128e122d387fe7
Reviewed-on: https://upspin-review.googlesource.com/8484
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/upspin,upspin/upspin