ref: Don't compute log strings if log level isn't satisfied.
ref: Don't compute log strings if log level isn't satisfied.

Before benchmark:
Benchmark____1B-12     30000      500408 ns/op     0.00 MB/s
After:
Benchmark____1B-12     30000      472023 ns/op     0.00 MB/s

Improvement of about 6%.

Change-Id: Ibb68cc74888db907759b4274a23cb6b4938deabd

vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref