Switch to zram swap

[MilesBHuff]

The system is under serious memory pressure even after fixing an overallocation in #244, as evidenced by the near-lack of page cache. The only way to really improve this situation is to increase swappiness, and that can only really happen if swap is in-memory: zram swap. (This issue supercedes #237 and #238.)

I will use the same settings I'm already successfully using elsewhere:

• zswap: 17%, lz4, zsmalloc (a hot cache, lightly compressed)
• zram swap: 33%, zstd-1 (a colder cache, moderately compressed)
• swappiness: 134 (swap is now faster than storage; page cache is now valuable)
• page-cluster: 0 (readahead on compressed RAM actually harms performance)

[MilesBHuff]

Completed in 2da423e.

[MilesBHuff]

I tested this with vmstat 1 while flying the distance from Tarn to Far-Easton in Delta (so, crossing the heaviest parts of the server). Swap behaved well, and there weren't any signs of thrashing, indicating that zswap is indeed successfully isolating hot pages from zram swap.

[MilesBHuff]

I just tested with watch -n1 cat /proc/pressure/memory while Elytra'ing at max speed from Tarn to Delta and around Delta extensively, yet there was very minimal memory pressure. The most I saw was the first time I loaded Delta, but that was very manageable, and after that the pressure was super-minimal.

For a server that is physically unable to run without swap, that is a very clean result.