schema/mysql: Use UNIX_TIMESTAMP() instead of CURRENT_TIMESTMAP()

`CURRENT_TIMESTAMP()` an alias for `NOW()` returns the current date and
time in the format `YYYY-MM-DD hh:mm:ss` using the session time zone.
Since we are using numeric context, the value is stored in the format
`YYYYMMDDhhmmss`. But actually we want to set a (millisecond) UNIX
timestamp here, so we need to use `UNIX_TIMESTAMP()` instead.

schema/mysql/schema.sql

@@ -1343,4 +1343,4 @@ CREATE TABLE icingadb_schema (
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin ROW_FORMAT=DYNAMIC;
 
 INSERT INTO icingadb_schema (version, timestamp)
-  VALUES (4, CURRENT_TIMESTAMP() * 1000);
+  VALUES (4, UNIX_TIMESTAMP() * 1000);

schema/mysql/upgrades/1.0.0-rc2.sql

@@ -156,7 +156,7 @@ ALTER TABLE acknowledgement_history
     MODIFY is_persistent enum('n','y') DEFAULT NULL COMMENT 'NULL if ack_set event happened before Icinga DB history recording';
 
 INSERT INTO icingadb_schema (version, timestamp)
-  VALUES (2, CURRENT_TIMESTAMP() * 1000);
+  VALUES (2, UNIX_TIMESTAMP() * 1000);
 
 ALTER TABLE host_state
     MODIFY output longtext DEFAULT NULL,

schema/mysql/upgrades/1.0.0.sql

@@ -288,4 +288,4 @@ INSERT INTO sla_history_downtime
   ON DUPLICATE KEY UPDATE sla_history_downtime.downtime_id = sla_history_downtime.downtime_id;
 
 INSERT INTO icingadb_schema (version, TIMESTAMP)
-  VALUES (3, CURRENT_TIMESTAMP() * 1000);
+  VALUES (3, UNIX_TIMESTAMP() * 1000);

schema/mysql/upgrades/1.1.1.sql

@@ -34,4 +34,4 @@ ALTER TABLE history
 UNLOCK TABLES;
 
 INSERT INTO icingadb_schema (version, timestamp)
-  VALUES (4, CURRENT_TIMESTAMP() * 1000);
+  VALUES (4, UNIX_TIMESTAMP() * 1000);