Add new tables for Event and Alarms. (sonic-net#852)

Why I did it
This PR contains code changes for providing extension to the Event Framework as specified in the sonic-net/SONiC#1409

How I did it
Followed design specified in sonic-net/SONiC#1409.
Add new tables for events and alarms.

common/schema.h

@@ -23,6 +23,7 @@ namespace swss {
 #define CHASSIS_APP_DB      12
 #define CHASSIS_STATE_DB    13
 #define APPL_STATE_DB       14
+#define EVENT_DB            19
 
 /***** APPLICATION DATABASE *****/
 
@@ -567,6 +568,12 @@ namespace swss {
 #define APP_FABRIC_MONITOR_DATA_TABLE_NAME	"FABRIC_MONITOR_TABLE"
 #define APP_FABRIC_MONITOR_PORT_TABLE_NAME	"FABRIC_PORT_TABLE"
 
+#define EVENT_HISTORY_TABLE_NAME          "EVENT"
+#define EVENT_CURRENT_ALARM_TABLE_NAME    "ALARM"
+#define EVENT_STATS_TABLE_NAME            "EVENT_STATS"
+#define EVENT_ALARM_STATS_TABLE_NAME      "ALARM_STATS"
+
+
 #ifdef __cplusplus
 }
 #endif