[ZmqOrch] Optimize memory by popping batch size at a time (#3910)

* [ZmqOrch] Optimize memory by popping batch size at a time
What I did

Used a reference instead of a unnecessary copy of a set object
Optimize memory by popping batch size at a time
NOTE: Please merge only after the below two PR's are merged

sonic-net/sonic-swss-common#1084
sonic-net/sonic-sairedis#1660

Author: vivekrnv

orchagent/dash/dashvnetorch.cpp

@@ -438,7 +438,7 @@ void DashVnetOrch::addPaValidation(const string& key, VnetMapBulkContext& ctxt)
     string underlay_ip_str = to_string(ctxt.metadata.underlay_ip());
     string pa_ref_key = ctxt.vnet_name + ":" + underlay_ip_str;
 
-    auto vnet_underlay_ips = vnet_table_[ctxt.vnet_name].underlay_ips;
+    auto& vnet_underlay_ips = vnet_table_[ctxt.vnet_name].underlay_ips;
     std::string underlay_sip_str = to_string(ctxt.metadata.underlay_ip());
     if (vnet_underlay_ips.find(underlay_sip_str) != vnet_underlay_ips.end())
     {

orchagent/zmqorch.cpp

@@ -9,14 +9,11 @@ void ZmqConsumer::execute()
 {
     SWSS_LOG_ENTER();
 
-    size_t update_size = 0;
     auto table = static_cast<swss::ZmqConsumerStateTable*>(getSelectable());
-    do
-    {
-        std::deque<KeyOpFieldsValuesTuple> entries;
-        table->pops(entries);
-        update_size = addToSync(entries);
-    } while (update_size != 0);
+
+    std::deque<KeyOpFieldsValuesTuple> entries;
+    table->pops(entries);
+    addToSync(entries);
 
     drain();
 }