infra: allow setting thread limits for the db haproxy

Sebastian Rieserequested to merge
feat/haproxy-thread-limits into devel

By default haproxy spawns one thread per CPU, this is not the right thing to do on nodes with lots of CPUs and lots of database services.

We set a reasonable default limit of 8 CPUs per haproxy and also allow setting the number of threads yourself (which should usually not be necessary).

Fixes #537 (closed).

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