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_stream/multi_session_test.cc

@@ -158,7 +158,7 @@ class MultiSessionTest : public ManifestTestBase {
     EXPECT_EQ(data->file_count, file_count);
     EXPECT_EQ(data->min_chunk_size, 128 << 10);
     EXPECT_EQ(data->avg_chunk_size, 256 << 10);
-    EXPECT_EQ(data->max_chunk_size, 1024 << 10);
+    EXPECT_EQ(data->max_chunk_size, 512 << 10);
   }
 
   metrics::ManifestUpdateData GetManifestUpdateData(