[#274] wc: Resolve possible deadlock

If operation with WC are _fast enough_ (e.g. `Init` failed and `Close` is
called immediately) there is a race and a deadlock that do not allow finish
(and start, in fact) an initialization routine because of taken `modeMtx`
and also do not allow finish `Close` call because of awaiting initialization
finish. So do stop initialization _before_ any mutex is taken.

Signed-off-by: Pavel Karpy <p.karpy@yadro.com>

pkg/local_object_storage/writecache/writecache.go

@@ -163,9 +163,6 @@ func (c *cache) Init() error {
 
 // Close closes db connection and stops services. Executes ObjectCounters.FlushAndClose op.
 func (c *cache) Close() error {
-	c.modeMtx.Lock()
-	defer c.modeMtx.Unlock()
-
 	// Finish all in-progress operations.
 	if err := c.setMode(context.TODO(), mode.ReadOnly); err != nil {
 		return err