Improve gravity parseLists on low-RAM systems

[DL6ER]

What does this implement/fix?

Use temp_store = FILE to avoid memory exhaustion for a very large number of lists in gravity parseLists. This is nothing else than extending the already merged pi-hole/pi-hole#5977 also to the parseList feature. I did not expect this to be affected, too, however, if single lists already have multi-million domains on them, then this is a problem is your Pi-hole is having only 512 MB total RAM (or even less...).

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Related issue or feature (if applicable): Fixes #2194

Pull request in docs with documentation (if applicable): N/A

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[rdwebdesign]

Looks like this is applied unconditionally in all cases.
Will this change make gravity slower in systems with enough memory? Or it doesn't matter?

[XhmikosR]

I don't think this is needed. The issue happens when creating the sqlite index. Not sure if this change hurts, but if you were to add this, shouldn't the same change be applied in all places?

Again, this doesn't help since the issue is when creating the index even on the development branch. I can reproduce with 256 RAM and ~2.400.000 domains. If I go down to around 1.400.00 domains RAM + ZRAM seems to be enough.

[DL6ER]

Superseded by #2321