Change fastcdc to a better and simpler algorithm. (#79)

This CL changes the chunking algorithm from "normalized chunking" to
simple "regression chunking", and changes the has criteria from
'hash&mask' to 'hash<=threshold'. These are all ideas taken from
testing and analysis done at
  https://github.com/dbaarda/rollsum-chunking/blob/master/RESULTS.rst
Regression chunking was introduced in
  https://www.usenix.org/system/files/conference/atc12/atc12-final293.pdf

The algorithm uses an arbitrary number of regressions using power-of-2
regression target lengths. This means we can use a simple bitmask for
the regression hash criteria.

Regression chunking yields high deduplication rates even for lower max
chunk sizes, so that the cdc_stream max chunk can be reduced to 512K
from 1024K. This fixes potential latency spikes from large chunks.

cdc_indexer/indexer.cc

@@ -140,8 +140,7 @@ Indexer::Impl::Impl(const IndexerConfig& cfg,
   fastcdc::Config ccfg(cfg_.min_chunk_size, cfg_.avg_chunk_size,
                        cfg_.max_chunk_size);
   Indexer::Chunker chunker(ccfg, nullptr);
-  cfg_.mask_s = chunker.Stage(0).mask;
-  cfg_.mask_l = chunker.Stage(chunker.StagesCount() - 1).mask;
+  cfg_.threshold = chunker.Threshold();
   // Collect inputs.
   for (auto it = inputs.begin(); it != inputs.end(); ++it) {
     inputs_.push(*it);
@@ -368,8 +367,7 @@ IndexerConfig::IndexerConfig()
       max_chunk_size(0),
       max_chunk_size_step(0),
       num_threads(0),
-      mask_s(0),
-      mask_l(0) {}
+      threshold(0) {}
 
 Indexer::Indexer() : impl_(nullptr) {}