dir/inprocess: only set dirContext with new, use to access store
dir/inprocess: only set dirContext with new, use to access store

The dirContext provided to inprocess.New should be used for
communicating with the storeserver, never the user's context, as the
dirserver is making requests on its own behalf, not for the user.
Clean up a few places where the wrong context was being used,
and make sure that the dirContext does not get clobbered by Dial.

Change-Id: Ie1577801cc719286580c70137cf94c26b4db6e9a
Reviewed-on: https://upspin-review.googlesource.com/5920
Reviewed-by: Eduardo Pinheiro <55040cbb39ba395339ae7940d8da6cd97797a4a8@google.com>
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@google.com>

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