indexserver: support SG_PRIORITY file for FS fake (#350)
This adds support for overriding the priority indexserver receives when
using the FS as the sourcegraph url.
Test Plan: Ran the following and indexserver updated the meta file with
the new priority
go run ./cmd/zoekt-sourcegraph-indexserver -sourcegraph_url ~/src/github.com/sourcegraph/
echo 100 > ~/src/github.com/sourcegraph/sourcegraph/SG_PRIORITY