Fix static field initialization from a non-static method in the virtualmap package

[thenswan]

This is a follow-up for #15562. After it will be merged, there will be static field initialization from a non-static method in the following classes:

1. VirtualNodeCache -- static getCleaningPool will be invoked from a non-static method and initialize cleaningPool.
2. VirtualHasher -- static getHashingPool will be invoked from a non-static method and initialize hashingPool.

(more detailed info will be in method-level docs)

The task is to refactor it so that there are no static fields but just instance fields.

[artemananiev]

One option to fix this is to wait till #14395 and make the pools above instance fields rather than static fields. There will be just on VirtualMap (per node), and it will have one hasher and one node cache.