Enable JSVM consumes a lot of memory

[lcx102737]

Branch/Environment/Version

• Branch/Version: v2.8.2
• Environment: centos7

Describe the bug
Enable JSVM consumes a lot of memory

Reproduction steps

1. enable JSVM
2. register api
3. top tyk

Actual behavior
The registration API calls jsvm.init, whether or not the API use js

Expected behavior
When register the compose API calls jsvm.init

code
The API type is not determined before calling spec.jsvm.init

func (a APIDefinitionLoader) MakeSpec(def *apidef.APIDefinition, logger *logrus.Entry) *APISpec {
	spec := &APISpec{}

	if logger == nil {
		logger = logrus.NewEntry(log)
	}

	......

	spec.GlobalConfig = config.Global()

	// Create and init the virtual Machine
	if config.Global().EnableJSVM {
		spec.JSVM.Init(spec, logger)
	}
       ......
}

Memory usage is as follows