Disable the feature that decreases link probe interval for measuring …

…switch overhead #49 (#54)

### Description of PR
Can't cleanly cherry pick the commit from master branch: 
34a68d1 disable switchover measuring based on link prober (#49)

Summary:
Fixes # (issue)

Disable part of the feature introduced in #43. 

The link probing interval will NOT be decreased by default. Link prober state change events will still be posted in `LINK_PROBE_STATS|PORTNAME` in state db. 

sign-off: Jing Zhang zhangjing@microsoft.com

### Type of change
- [x] New feature

### Approach
#### What is the motivation for this PR?
We need to reconsider the design of this feature. 

To be more specific, this is a special case of decreasing probing interval, it's for measurement purposes only. We still want to trigger the toggle in 300ms when pack loss happens. The negative count should be 30 instead of 3 when interval is decreased to 10ms.

src/common/MuxPortConfig.h

@@ -260,13 +260,24 @@ class MuxPortConfig
     */
     inline uint32_t getDecreasedTimeoutIpv4_msec() const {return mMuxConfig.getDecreasedTimeoutIpv4_msec();};
 
+    /**
+     * @method ifEnableSwitchoverMeasurement
+     * 
+     * @brief check if the feature that decreases link prober interval to measure switch overhead is enabled or not 
+     * 
+     * @return if switch overhead measurement feature is enabled
+     */
+    inline bool ifEnableSwitchoverMeasurement() {return mEnableSwitchoverMeasurement;};
+
 private:
     MuxConfig &mMuxConfig;
     std::string mPortName;
     boost::asio::ip::address mBladeIpv4Address;
     std::array<uint8_t, ETHER_ADDR_LEN> mBladeMacAddress = {0, 0, 0, 0, 0, 0};
     uint16_t mServerId;
     Mode mMode = Manual;
+
+    bool mEnableSwitchoverMeasurement = false;
 };
 
 } /* namespace common */

src/link_manager/LinkManagerStateMachine.cpp

@@ -363,7 +363,9 @@ void LinkManagerStateMachine::switchMuxState(
         mMuxStateMachine.setWaitStateCause(mux_state::WaitState::WaitStateCause::SwssUpdate);
         mMuxPortPtr->postMetricsEvent(Metrics::SwitchingStart, label);
         mMuxPortPtr->setMuxState(label);
-        mDecreaseIntervalFnPtr(mMuxPortConfig.getLinkWaitTimeout_msec()); 
+        if(mMuxPortConfig.ifEnableSwitchoverMeasurement()) {
+            mDecreaseIntervalFnPtr(mMuxPortConfig.getLinkWaitTimeout_msec()); 
+        }
         mDeadlineTimer.cancel();
         startMuxWaitTimer();
     } else {