Reduce WIAB memory footprint

The excessive footprint can be avoided if the wave server evicts the oldest deltas from memory, that is, it only keeps the most recent deltas in memory and loads older deltas from disk only upon request. Since we don't want to perform any disk operations synchronously under the wavelet lock in the wave store, we need to make all the methods that access delta history asynchronous. Specifically, the following methods need to be made asynchronous:
box.server.waveserver.WaveletContainer#requestHistory()
box.server.waveserver.WaveletContainer#requestTransformedHistory()
box.server.waveserver.WaveletProvider#getHistory()