[korotin]

LoadMany spawns a new goroutine per key, which may lead to excessive CPU and memory usage under heavy load.

Since LoadMany is just a wrapper around Load calls, and Load itself is guarded by two global locks (batchLock and cacheLock), the goroutines don't seem to buy us anything here and, as far as I can tell, can be removed entirely with no functional drawbacks.

In my PR I've refactored LoadMany to get rid of those goroutines.

I believe the change is backwards compatible, but there's one small detail worth mentioning: LoadMany used to call thunk() itself, which is no longer the case — this makes LoadMany lazy, just like Load already is. I'd expect this to be a minor change that shouldn't break anything, but please let me know if I'm missing a case here.