Use portable CRC32C from google/crc32c.
Use portable CRC32C from google/crc32c.

Benchmark results below. More results at
https://github.com/google/crc32c/commit/354d61ef97eb73d4f0c1694cac6ec6116a5543e4.

New, MacBookPro13,3 with Core i7 6920HQ:
LevelDB:    version 1.20
Keys:       16 bytes each
Values:     100 bytes each (50 bytes after compression)
Entries:    1000000
RawSize:    110.6 MB (estimated)
FileSize:   62.9 MB (estimated)
WARNING: Snappy compression is not enabled
------------------------------------------------
fillseq      :       2.952 micros/op;   37.5 MB/s
fillsync     :      43.932 micros/op;    2.5 MB/s (1000 ops)
fillrandom   :       3.856 micros/op;   28.7 MB/s
overwrite    :       4.053 micros/op;   27.3 MB/s
readrandom   :       4.234 micros/op; (1000000 of 1000000 found)
readrandom   :       3.923 micros/op; (1000000 of 1000000 found)
readseq      :       0.201 micros/op;  550.8 MB/s
readreverse  :       0.356 micros/op;  310.6 MB/s
compact      :  436800.000 micros/op;
readrandom   :       2.375 micros/op; (1000000 of 1000000 found)
readseq      :       0.151 micros/op;  734.3 MB/s
readreverse  :       0.298 micros/op;  370.7 MB/s
fill100K     :     554.075 micros/op;  172.1 MB/s (1000 ops)
crc32c       :       1.393 micros/op; 2805.0 MB/s (4K per op)
snappycomp   :    3902.000 micros/op; (snappy failure)
snappyuncomp :    3821.000 micros/op; (snappy failure)
acquireload  :      13.088 micros/op; (each op is 1000 loads)

Baseline, MacBookPro13,3 with Core i7 6920HQ:
LevelDB:    version 1.20
Keys:       16 bytes each
Values:     100 bytes each (50 bytes after compression)
Entries:    1000000
RawSize:    110.6 MB (estimated)
FileSize:   62.9 MB (estimated)
WARNING: Snappy compression is not enabled
------------------------------------------------
fillseq      :       3.000 micros/op;   36.9 MB/s
fillsync     :      46.721 micros/op;    2.4 MB/s (1000 ops)
fillrandom   :       3.922 micros/op;   28.2 MB/s
overwrite    :       4.080 micros/op;   27.1 MB/s
readrandom   :       4.409 micros/op; (1000000 of 1000000 found)
readrandom   :       3.895 micros/op; (1000000 of 1000000 found)
readseq      :       0.190 micros/op;  582.4 MB/s
readreverse  :       0.413 micros/op;  267.6 MB/s
compact      :  441076.000 micros/op;
readrandom   :       2.308 micros/op; (1000000 of 1000000 found)
readseq      :       0.170 micros/op;  651.2 MB/s
readreverse  :       0.302 micros/op;  366.2 MB/s
fill100K     :     614.289 micros/op;  155.3 MB/s (1000 ops)
crc32c       :       3.547 micros/op; 1101.2 MB/s (4K per op)
snappycomp   :    3393.000 micros/op; (snappy failure)
snappyuncomp :    3171.000 micros/op; (snappy failure)
acquireload  :      12.761 micros/op; (each op is 1000 loads)

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=170100372

cmumford/leveldb,fastogt/leveldb,olzhabay/PMIndexDB,matehat/leveldb,likema/leveldb,dwing4g/leveldb,cmumford/leveldb,matehat/leveldb,olzhabay/PMIndexDB,fastogt/leveldb,fuchsia-mirror/third_party-leveldb,google/leveldb,dwing4g/leveldb,dwing4g/leveldb,likema/leveldb,yuwolf/leveldb,bitcoin-core/leveldb,luke-jr/leveldb,yuwolf/leveldb,bitcoin-core/leveldb,matehat/leveldb,olzhabay/PMIndexDB,luke-jr/leveldb,yuwolf/leveldb,google/leveldb,bitcoin-core/leveldb,fuchsia-mirror/third_party-leveldb