Preallocate fstree directories

[roman-khimov]

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

I'm always frustrated when we stat/mkdir during object put.

Describe the solution you'd like

I think we can preallocate to depth 2 right at fstree init (just a bit more than 3K directories). Theoretically depth 3 can be done in background, but maybe it's not worth it, needs to be tested. If we're to perform any background directory creation fstree config can be leveraged to mark tree as completely ready (allowing to avoid directory stat), see #2174 (comment).

Describe alternatives you've considered

Things work as is.