Link + strip binaries in parallel

The FDB binaries are being linked + stripped one at a time by the Ninja. To reproduce:
```bash
ninja clean
ninja strip_targets # builds all, then all the binaries, then strips them.
```
and watch top at the end of the build process.

With ccache and distcc, the linking and stripping step dominates the compilation time for incremental builds.

Start by locating the Ninja rules and dependency graph behind `strip_targets`, then reproduce with `ninja clean` followed by `ninja strip_targets` while observing the final build in `top`. Verify that linking and stripping independent FDB binaries can run concurrently and that the target still completes correctly with ccache and distcc.

Written by the indexing model from the issue text.