Reduce number of locks and buckets generated by BucketBasedCacheStore

The current implementation of FileCacheStore creates one bucket and lock for each hash key - which results in up to 4.2 billion files (2^32).

It should limit the number of files to
a) improve performance of purge
b) reduce number of open file handles (system resources)
c) reduce number of Java objects/heap (JVM resources)
d) improve performance

The implementation allows us to do so.

Only 4 lines of code are necessary in BucketBasedCacheStore implementation:

   private int hashKeyMask = 0xfffffc00; // TODO should get a configuration entry

   @Override
   protected Integer getLockFromKey(Object key) {
      return Integer.valueOf(key.hashCode() & hashKeyMask);
   }

This reduces the number of files to 2^22 = 4,194,304 files. Since each application and each cache store has different semantics the hasKeyMask value should be configurable - best would be to configure the number of bits.

Side effect: If someone changes the FileCacheStore hasKeyMask, the whole cache store becomes unuseable. So I opened another enhancement ...

Note: This implementation should be used in a different class (e.g. extend FileCacheStore) because it makes existing file cache stores unusable.