Add SonicDBConfig::reset method (sonic-net#843)

#### Why I did it
sonic-gnmi will use swsscommon API to read database configuration, and sonic-gnmi unit test needs to reload database configuration in the same process.

#### How I did it
Add SonicDBConfig::reset method, and sonic-gnmi unit test will use this method to clear previous database configuration.

#### How to verify it
Pass all UT and E2E test cases.

### Description for the changelog
Add SonicDBConfig::reset method

common/dbconnector.cpp

@@ -185,6 +185,18 @@ void SonicDBConfig::initialize(const string &file)
     m_init = true;
 }
 
+// This API is used to reset the SonicDBConfig class.
+// And then user can call initialize with different config file.
+void SonicDBConfig::reset()
+{
+    std::lock_guard<std::recursive_mutex> guard(m_db_info_mutex);
+    m_init = false;
+    m_global_init = false;
+    m_inst_info.clear();
+    m_db_info.clear();
+    m_db_separator.clear();
+}
+
 void SonicDBConfig::validateNamespace(const string &netns)
 {
     std::lock_guard<std::recursive_mutex> guard(m_db_info_mutex);
@@ -927,4 +939,4 @@ map<string, map<string, map<string, string>>> DBConnector::getall()
         }
     }
     return data;
-}
+}

common/dbconnector.h

@@ -59,6 +59,7 @@ class SonicDBConfig
             SonicDBConfig.initializeGlobalConfig(global_db_file_path)
     %}
 #endif
+    static void reset();
 
     static void validateNamespace(const std::string &netns);
     static std::string getDbInst(const std::string &dbName, const std::string &netns = EMPTY_NAMESPACE);

tests/main.cpp

@@ -70,6 +70,19 @@ class SwsscommonEnvironment : public ::testing::Environment {
             EXPECT_TRUE(strstr(e.what(), "Namespace invalid is not a valid namespace name in config file"));
         }
 
+        // reset SonicDBConfig, init should be false
+        SonicDBConfig::reset();
+        cout<<"RESET: isInit = "<<SonicDBConfig::isInit()<<endl;
+        EXPECT_FALSE(SonicDBConfig::isInit());
+        EXPECT_FALSE(SonicDBConfig::isGlobalInit());
+
+        // reinitialize SonicDBConfig, init should be true
+        SonicDBConfig::initialize(existing_file);
+        cout<<"INIT: load local db config file, isInit = "<<SonicDBConfig::isInit()<<endl;
+        EXPECT_TRUE(SonicDBConfig::isInit());
+        SonicDBConfig::initializeGlobalConfig(global_existing_file);
+        cout<<"INIT: load global db config file, isInit = "<<SonicDBConfig::isGlobalInit()<<endl;
+        EXPECT_TRUE(SonicDBConfig::isGlobalInit());
     }
 };