SEARCHER_URL, SYMBOLS_URL should allow specifying comma separated list

Created by: slimsag

Currently, SEARCHER_URL may only specify multiple values by using the k8s+ syntax which utilizes Kubernetes service discovery (effectively).

In non-Kubernetes data center deployments, such as a pure-docker one, we need to be able to specify multiple via the environment variable. e.g.:

SEARCHER_URL=http://searcher-0:3181,http://searcher-1:3181,http://searcher-2:3181

https://github.com/sourcegraph/sourcegraph/blob/3d3ffe9c2a975b4dd2e623bfa16bb29d3c6ecde9/cmd/frontend/graphqlbackend/textsearch.go#L931

The same must be fixed for SYMBOLS_URL:

https://github.com/sourcegraph/sourcegraph/blob/3d3ffe9c2a975b4dd2e623bfa16bb29d3c6ecde9/pkg/symbols/client.go#L26