[MINOR] check: add redis check support

This patch provides a new "option redis-check" statement to enable server health checks based on redis PING request (http://www.redis.io/commands/ping).
diff --git a/include/common/defaults.h b/include/common/defaults.h
index 96e0f61..0a4f420 100644
--- a/include/common/defaults.h
+++ b/include/common/defaults.h
@@ -131,6 +131,7 @@
 #define DEF_CHECK_REQ   "OPTIONS / HTTP/1.0\r\n"
 #define DEF_SMTP_CHECK_REQ   "HELO localhost\r\n"
 #define DEF_LDAP_CHECK_REQ   "\x30\x0c\x02\x01\x01\x60\x07\x02\x01\x03\x04\x00\x80\x00"
+#define DEF_REDIS_CHECK_REQ  "*1\r\n$4\r\nPING\r\n"
 
 #define DEF_HANA_ONERR		HANA_ONERR_FAILCHK
 #define DEF_HANA_ERRLIMIT	10