Add expirable LRU finalizer to fix expirable LRU's goroutine leak

[canoriz]

Apply the trick used in go-cache to solve go routine leak when dynamically creating many expirable-LRU cache. #159

The deleteExpired go routine will be terminated by finalizer at some garbage collection time in the future, although not immediately after cache goes out of scope.

This trick is wrapping the lru type and does not affect exposed APIs. If LRU.Close() method will be re-added in the future, a small change is needed to prevent double close over c.done. But for now, the only caller of close(c.done) is the finalizer, and the finalizer must be called only once, so it's OK for now.

[hashicorp-cla]

All committers have signed the CLA.

[canoriz]

Could you please take a look at these codes when you have some time? @paskal

[paskal]

Clever idea. I like it.

[paskal]

I've had time to think about it, and now I think it's better to make it simply a "Close" method rather than destructor magic. Using a destructor in Go feels too clever.

[canoriz]

I agree close() is better. But before close is re-added, create cache (especially inside a for loop) leaks go routines.