Add support to the Java SDK harness to merge windows

Window merging function:

Input: KV<nonce, iterable<OriginalWindow>>
Output: KV<nonce, KV<iterable<UnmergedOriginalWindow>, iterable<KV<MergedWindow, iterable<ConsumedOriginalWindow>>>> 

For each set of original windows, a list of all unmerged windows is output alongside a map of merged window to set of consumed windows. All original windows must be contained in either the unmerged original window set or one of the consumed original window sets. Each original window can only be part of one output set. The nonce is used by a runner to associate each input with its output. The nonce is represented as an opaque set of bytes.