rework CacheWriter writeAll() and deleteAll() to permit partial success
Merge pull request #204 from timeck/cacheWriterChanges

rework CacheWriter writeAll() and deleteAll() to permit partial success
Issue #191 Basic getAll signature change
Issue #191 Change Store.bulkComputeIfAbsent signature
Issue #191 Change Cache.getAll contract: return all keys, mapped to value or null if not in cache
Issue #191 Change Cache.putAll signature: input param is now Map<? extends K, ? extends V>
Issue #191 Change Cache.removeAll signature: input param is now Set<? extends K>
Issue #191 Change Store.bulkCompute signature: first input param is now Set<? extends K>
Issue #191 - deep null check for getAll(Set<K>) and removeAll(Set<K>)