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/README.md

@@ -14,7 +14,7 @@ experimentation. See the file `indexer.h` for preprocessor macros that can be
 enabled, for example:
 
 ```
-bazel build -c opt --copt=-DCDC_GEAR_TABLE=1 //cdc_indexer
+bazel build -c opt --copt=-DCDC_GEAR_BITS=32 //cdc_indexer
 ```
 
 At the end of the operation, the indexer outputs a summary of the results such
@@ -25,7 +25,7 @@ as the following:
 Operation succeeded.
 
 Chunk size (min/avg/max): 128 KB / 256 KB / 1024 KB  |  Threads: 12
-gear_table: 64 bit  |  mask_s: 0x49249249249249  |  mask_l: 0x1249249249
+gear_table: 64 bit  |  threshold: 0x7fffc0001fff
            Duration:           00:03
         Total files:               2
        Total chunks:           39203