[#947] writecache: refactor object persisting

a1696a8 introduced some logic which in some situations prevented big objects
to be persisted in FSTree. In this commit a refactoring is done with the
goal of simplifying the code and also checking #866 issue.

1. Split a monstrous function into multiple simple ones: memory objects
   can only be small and for writing through the cache we can do a dispatch
   in `Put` itself.
2. Determine objects to be put in database before the actual update
   as setting up a transaction has non-zero overhead.

Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>

pkg/local_object_storage/writecache/writecache.go

@@ -6,7 +6,6 @@ import (
 	objectSDK "github.com/nspcc-dev/neofs-api-go/pkg/object"
 	"github.com/nspcc-dev/neofs-node/pkg/core/object"
 	"github.com/nspcc-dev/neofs-node/pkg/local_object_storage/blobstor/fstree"
-	"go.uber.org/atomic"
 	"go.uber.org/zap"
 )
 
@@ -44,8 +43,6 @@ type cache struct {
 	evictCh chan []byte
 	// store contains underlying database.
 	store
-	// dbSize stores approximate database size. It is updated every flush/persist cycle.
-	dbSize atomic.Uint64
 	// fsTree contains big files stored directly on file-system.
 	fsTree *fstree.FSTree
 }