Allow parallel Bazel invocations

The time spent waiting for another Bazel command seems wasted. Can Bazel accomplish some work in tandem with the other command since Bazel uses a server daemon?

In particular, can `bazel build //some:target` and `bazel build //some/other:target` have their action graphs joined together and each separate command finish when its requested targets are completed?

However, at the very least, can Bazel show the progress of the other command?

Start by investigating Bazel's server-daemon handling of concurrent invocations and how action graphs are built for separate build commands. Compare the requested behaviors: joining action graphs so each command finishes at its targets, or at minimum showing progress from the other command; done means one of these behaviors is implemented and verified.

Written by the indexing model from the issue text.