migrate: Refactor _lockMigrations to avoid forUpdate

Since _isLocked is only called just before _lockMigrations, it is redundant
and we can accomplish the same  thing by checking the rowCount returned
by the update (verified by testing with postgres, mysql, sqlite, and
mssql).

The benefit of this change is improving compatbility with CockroachDB,
which does not support FOR UPDATE.

Updates knex#2002

lib/migrate/Migrator.js

@@ -240,32 +240,23 @@ class Migrator {
     }
   }
 
-  _isLocked(trx) {
-    const tableName = getLockTableName(this.config.tableName);
-    return getTable(this.knex, tableName, this.config.schemaName)
-      .transacting(trx)
-      .forUpdate()
-      .select('*')
-      .then((data) => data[0].is_locked);
-  }
-
   _lockMigrations(trx) {
     const tableName = getLockTableName(this.config.tableName);
     return getTable(this.knex, tableName, this.config.schemaName)
       .transacting(trx)
-      .update({ is_locked: 1 });
+      .where('is_locked', '=', 0)
+      .update({ is_locked: 1 })
+      .then((rowCount) => {
+        if (rowCount != 1) {
+          throw new Error('Migration table is already locked');
+        }
+      });
   }
 
   _getLock(trx) {
     const transact = trx ? (fn) => fn(trx) : (fn) => this.knex.transaction(fn);
     return transact((trx) => {
-      return this._isLocked(trx)
-        .then((isLocked) => {
-          if (isLocked) {
-            throw new Error('Migration table is already locked');
-          }
-        })
-        .then(() => this._lockMigrations(trx));
+      return this._lockMigrations(trx);
     }).catch((err) => {
       throw new LockError(err.message);
     });