Stick to convention for factory functions

[Enmk]

According to Go's naming convention factory function should be NewUserStore instead of InitUserStore
https://golang.org/doc/effective_go.html#composite_literals

https://github.com/Appscrunch/Multy-back/blob/d11d67f6b099529f9ce76bf9695493e6fb1c3cf3/store/store.go#L61