Added mempool inSync property to backend. Display on frontend when not in sync and on't create statistics when not in sync.

fixes #29

frontend/src/app/components/footer/footer.component.html

@@ -3,9 +3,14 @@
     <div class="row text-center" *ngIf="memPoolInfo">
       <div class="col d-none d-sm-block">
         <span class="txPerSecond">Tx weight per second:</span>
-        <div class="progress sub-text">
-          <div class="progress-bar {{ progressClass }}" role="progressbar" [ngStyle]="{'width': progressWidth}">{{ memPoolInfo?.vBytesPerSecond | ceil | number }} vBytes/s</div>
-        </div>      
+        <span class="" *ngIf="memPoolInfo?.vBytesPerSecond === 0; else inSync">
+          &nbsp;<span class="badge badge-pill badge-warning">Backend is synchronizing</span>
+        </span>
+        <ng-template #inSync>
+          <div class="progress sub-text">
+            <div class="progress-bar {{ progressClass }}" role="progressbar" [ngStyle]="{'width': progressWidth}">{{ memPoolInfo?.vBytesPerSecond | ceil | number }} vBytes/s</div>
+          </div>
+        </ng-template>    
       </div>
       <div class="col">
         <span class="unconfirmedTx">Unconfirmed<span class="extra-text"> transactions</span>:</span>