Concurrent calls to retrieveProductsInfo cause a crash

## Bug Report
Concurrent calls to retrieveProductsInfo cause a crash

## Platform Information
- OS: ALL
- Purchase Type: ALL
- Environment: ALL
- SwiftyStoreKit version: 0.16.1

A spinlock should be added to the inflightRequests property on write?

`
func testAppleStoreKit() {
let productIds :Set = ["productId"]
DispatchQueue.concurrentPerform(iterations: 1_000) { idx in
SwiftyStoreKit.retrieveProductsInfo(productIds) { results in
print(idx)
}
}
}
`