'Bindings' has concurrent r/w issue in DSL sample

[zyingfei]

https://github.com/samarabbas/cadence-samples/blob/c65d2c53058c678e4b98fedf2f76560f83bfd9dc/cmd/samples/dsl/workflow.go#L120

Seems map will panic if 'bindings' is read at the same time.