daemon startup is a bit slow

I took an existing Bazel monorepo I work on and started experimenting with BUCK as a learning exercise. I was quite surprised that the daemon takes ~5 seconds to startup:

```
time buck2 clean && time buck2 targets ...
killing buckd server
Buck2 daemon pid 2029993 has exited
/PATHTOREPO/buck-out/v2/gen
/PATHTOREPO/buck-out/v2/log
/PATHTOREPO/buck-out/v2/tmp
/PATHTOREPO/buck-out/v2/forkserver
/home/robertc/.buck/buckd//PATHTOREPO/v2

real 0m0.175s
user 0m0.018s
sys 0m0.023s
Build ID: de17d116-cd54-4890-b50c-bbbbb15da49d
Jobs completed: 8. Time elapsed: 0.3s.
root//:hello_world

real 0m4.994s
user 0m0.006s
sys 0m0.035s
```

The repo is very modest in size:
```
time find . >& /dev/null

real 0m0.161s
user 0m0.018s
sys 0m0.144s
```

Even forcing a device check - so statting:
```
time du -x >& /dev/null

real 0m0.237s
user 0m0.013s
sys 0m0.224s
```

It is obviously not a terrible thing - 5 seconds once a while - but it is a surprise given the emphasis on speed in the documentation.