scripts: Add clean-hashserver-database script

scripts: Add clean-hashserver-database script

Auxiliary script to clean the hashserver database based on the files
available in the sstate directory.

It makes used of the new "hashclient gc-mark-stream" command to mark all sstate
relevant hashes as "alive" and removes everything else from the
database.

Usage example:
```
./scripts/clean-hashserver-database \
    --sstate-dir ~/build/sstate-cache \
    --hashclient ./bitbake/bin/bitabke-hashclient \
    --hashserver-address "ws://localhost:8688/ws" \
    --mark "alive" \
    --threshold-age 60 \
    --clean-db
```

Signed-off-by: Alexander Marques <c137.marques@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>