Add the ability to fetch a set of keys at once (getAll)

When a transaction knows in advance about the set of keys it needs to read, this cache.getAll(k1,k2,..kn) method can bring a some performance improvement:
1. all the keys can be fetched from remote nodes in parallel
2. keys that map to the same node can be grouped and fetched in the same RPC request

Note that 1. can be be achieved even at this time by using Cache.getAsync(K) method - that's not as elegant though as it requires the user to write the code the code that waits on the Future objects that are returned.