Add a check for warm-restart, and do a clear only when warm-restart is enable. (sonic-net#1498)

This commit is to address the issue that the NEIGH_TABLE loaded by swssconfig
after fast-reboot is cleared by neighsyncd.

**What I did**
Fix sonic-net#5841 and sonic-net#5580

We found that neighbor table loaded by ```swssconfig``` from ```arp.json``` after ```fast-reboot``` is cleared by ```neighsyncd``` mistakenly at the initial stage. This PR adds a check for ```WarmStart``` before cleaning up, and only do that if ```WarmStart``` is enable.

**Why I did it**
This PR is to fix the issue that arp table is not recovered after fast-reboot.

**How I verified it**
Verified on Arista-7260, running 201911 image.
1. Run some test to populate ARP entries on DUT, such as ```test_fast_reboot```
2. Issue a fast-reboot
3. Verify the ```arp.json``` backed up by ```fast-reboot-dump.py``` is loaded and NEIGH_TABLE is restored.

Author: bingwang-ms

warmrestart/warmRestartAssist.cpp

@@ -79,7 +79,10 @@ void AppRestartAssist::registerAppTable(const std::string &tableName, ProducerSt
     m_psTables[tableName]  = psTable;
 
     // Clear the producerstate table to make sure no pending data for the AppTable
-    psTable->clear();
+    if (m_warmStartInProgress)
+    {
+        psTable->clear();
+    }
     m_appTables[tableName] = new Table(m_pipeLine, tableName, false);
 }
 
@@ -150,7 +153,7 @@ void AppRestartAssist::readTablesToMap()
     }
     return;
 }
- 
+
 /*
  * Check and insert to CacheMap Logic:
  * if delete_key: