zoekt-webserver: introduce sourcegraph/log (#411)
This adds a single use case of sourcegraph/log, the standard structured
logger at Sourcegraph. Right now it will just output our per search
logs. The intention is to slowly convert this to only use
sourcegraph/log.
For now I am using a named import for sglog since converting all the
existing uses of stdlib log would make this a very large commit.
Test Plan: SRC_LOG_LEVEL=info go run ./cmd/zoekt-webserver