Performance improvements and instrumentation for snapshot

* Deflate snapshots using stream API
* Make LogManager non-final

Testing Done:
./gradlew -Pq build

The StreamManager @Timed annotations don't work yet since guice
isn't used to instantiate it. Ideally we'd use AssistedInject here,
but that's a slightly larger change.

Bugs closed: AURORA-722

Reviewed at https://reviews.apache.org/r/25760/