Engine-level write cache

[roman-khimov]

Is your feature request related to a problem? Please describe.

I'm always frustrated when I'm configuring per-shard write caches. They're powerful, but they're not very convenient, practically we always have a single disk for them. These caches are only relevant in case they're faster than the primary shard medium (HDD vs SSD), but we never have and not very likely to have an SSD for each HDD in a single server (this suggests building an SSD-only solution in the first place which doesn't need any write cache at all).

Describe the solution you'd like

Make write cache engine-level, not shard-level.

Describe alternatives you've considered

Keep things as is. It works too.

Additional context

#2337.