remove old builds from memory

[TheJJ]

all builds are stored on disk anyway, but we should drop old builds from ram.
this can prevent denial of service attacks where all builds for projects are walked through and requested, thereby loaded from disk into ram. for many builds, this can exhaust memory of the process.

-> keep a maximum of $n builds in memory, delete more.

[TheJJ]

We now have the BuildManager for that. It still doesn't delete old builds from memory, but it could easily do that now.