Classify using token prefix

For gitlab-org/gitlab#465432

The changes in this MR supports

• renames the attribute named session_prefix in config.toml to just prefix, because "session_prefix" is too specific. We would use the same prefix to classify a cell, whether that data originates from session or a token, so it made sense to just use "prefix" as the attribute name.

• classification using a token's prefix.

  • A token's prefix matches a cell's prefix attribute specified in the configuration, so we match this value (which HTTP router would pass to TS) in the request to the prefix attributes in the TS's configuration to find the right cell to route the request to.

With a valid cell name

curl --cacert tmp/certs/ca-cert.pem --cert tmp/certs/client-cert.pem --key tmp/certs/client-key.pem  -X POST https://localhost:9096/v1/classify -H "Content-Type: application/json"   -d '{"type": "TokenPrefix", "value": "cell-1"}'

{"action":"Proxy","proxy":{"address":"my.cell-1.example.com"}}

With an invalid cell name

curl --cacert tmp/certs/ca-cert.pem --cert tmp/certs/client-cert.pem --key tmp/certs/client-key.pem  -X POST https://localhost:9096/v1/classify -H "Content-Type: application/json"   -d '{"type": "TokenPrefix", "value": "junk_value"}'

{"code":5,"message":"cell not found: \"junk_value\"","details":[]}