speed up resolve2(uid) on the critical path of KBFS rekeys (#6161)
speed up resolve2(uid) on the critical path of KBFS rekeys (#6161)

- use the UPAK LookupUsername rather than going to the server
- and single-flight resolutions
keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client