Allow a non-String object as a locking key in LockSupportCacheStore

LockSupportCacheStore uses String as its locking key. It is OK as long as the locking key can be represented as a String without much conversion cost. However, it the extending CacheStore implementation uses other type such as byte[] as its primary key type, it will result in unnecessary conversion from byte[] to String to achieve fine grained locking. I found a similar problem in BucketBasedCacheStore - it's converting an integer into a String unnecessarily.

Therefore, I propose to generify LockSupportCacheStore:

/**
 * @param L the type of the locking key
 */
public abstract class LockSupportCacheStore<L> ... {
   ...
   protected abstract String getLockFromKey(L key) throws CacheLoaderException;
   ...
}

Then BucketBasedCacheStore and the new CacheStore implementation I'm writing for ISPN-701 will be more efficient.

Mircea, could you confirm if my idea doesn't break anything? Because it's not a backward compatible change, I'd like to get this done before cutting the first candidate release.