Move in-memory db to a file-based hsql db

[mauricioaniche]

The process metrics database is, right now, a simple in-memory HashMap. That works, but for large projects, this map might become too big to fit in our small VMs.

One idea is to move it to a file-based HSQLDB database. After all, we often have more disk than memory available.

We should make sure to reset the file whenever the application starts (a force reset at the beginning, as the previous execution of application might not had ended gracefully..)

[jan-gerling]

@mauricioaniche Can we close this issue, as it is no longer relevant?