store/storecache: use 503 as a named const
store/storecache: use 503 as a named const

Make the code easier to follow.

Change-Id: I6a7d12a66e63680bb9a8eb6deffd2e2bd294f325
Reviewed-on: https://upspin-review.googlesource.com/8557
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/upspin,upspin/upspin