Upgrade to Cabal-2.0

[hvr]

hackage-server is currently Cabal-1.24 based; but we need to upgrade it to Cabal-2.0 real soon. This is expected to provide substantial heap memory savings.

[hvr]

The heap-profiles below show the startup phase of hackage and then idling for a little amount of time.

Cabal 1.24 Heap-profile

Cabal 2.0 Heap-profile

---

This has been enabled by data-representation optimizations in Cabal (most notably haskell/cabal#3896 and haskell/cabal#3905).

This results in a reduction of about 0.5 GiB worth of (:) heap-objects, which consequently means a reduction of about ~20 million (:) allocated on the heap the GC won't have to traverse anymore! Overall, in terms of actual process-size (i.e. RSS), migrating to Cabal-2.0 translates into saving about 2 GiB of RAM.

TODO: figure out what those (:)-allocation spikes are which are still present late into the startup.