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conf: cache getMode

Administrator requested to merge k/20220902-cache-getMode into main

Created by: keegancsmith

getMode() is called everytime we call conf.Get(). The getMode implementation calls os.Getenv which requires a system call. However, the result of getMode will never change over the lifetime of a process. So we can cache its result and avoid the system call.

This was noticed while benchmarking sub repository permissions. Currently the code for that calls conf.Get() per file in a repository. This can be changed, but what it did show this performance issue. Here is the impact on that benchmark:

name                 old time/op  new time/op   delta
FilterActorPaths-32  52.4ms ± 7%    3.9ms ±10%    -92.60%  (p=0.000 n=10+10)

name                 old paths/s  new paths/s   delta
FilterActorPaths-32   95.7k ± 8%  1294.0k ±11%  +1251.45%  (p=0.000 n=10+10)

Benchmark is here https://github.com/sourcegraph/sourcegraph/pull/41257

Test Plan: go test

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