Remove redundant async call in FRU collection threads (ibm-openbmc#588)

This commit removes the redundant std:async call in the detached thread
launched for parsing and publishing the VPD for an individual FRU.
Since we have a dedicated detached thread for each FRU, we can do VPD
parse and publish in a synchronous manner from the detached thread
itself, instead of launching a asynchronous task which adds unnecessary
performance cost.

This commit addresses issue ibm-openbmc#558.

Test:

```
1. Install bitbaked image on Everest (ever6bmc)
2. After initial boot, check:
	- BMC State Ready
	- vpd-manager's CollectionStatus property should be "Completed"
	busctl get-property com.ibm.VPD.Manager
	/com/ibm/VPD/Manager com.ibm.VPD.Manager
	CollectionStatus
	s "Completed"
	- vpd-manager status should be running with no restarts
	- vpd-manager should have only 1 thread running:
	  check "ls -la /proc/<vpd-manager PID>/task"
3. Reboot the BMC several times and repeat Step 2.
```

Change-Id: I603c64dc9b5057429a2288f0edfde6086755b851
Signed-off-by: Souvik Roy <[email protected]>

vpd-manager/src/worker.cpp

@@ -1513,12 +1513,8 @@ void Worker::collectFrusFromJson()
         }
 
         std::thread{[vpdFilePath, this]() {
-            auto l_futureObject = std::async(&Worker::parseAndPublishVPD, this,
-                                             vpdFilePath);
+            const auto& l_parseResult = parseAndPublishVPD(vpdFilePath);
 
-            std::tuple<bool, std::string> l_threadInfo = l_futureObject.get();
-
-            // thread returned.
             m_mutex.lock();
             m_activeCollectionThreadCount--;
             m_mutex.unlock();