Monorepo: Optimize CI Builds

The goal is to get the PR builds to < 20m, including the -pubsub and -spanner libraries.

In general we need to reduce the build times by:

• Avoid building things that have not changed, use caching to let the build system take care of it.
• Avoid running tests that have not changed
• Parallelize unit tests, integration tests, examples and benchmarks, particularly any "slow" ones to reduce latency
• Prefer "fast build" modes for PRs, leaving slow builds (e.g. -O2 builds) for post-PR