Snapshot optimizations

[rphmeier]

Here are some numbers/logs:

old:

$ parity snapshot ~/1.3.0.snap
2016-08-23 18:54:01 Configured for Frontier/Homestead using Ethash engine
2016-08-23 18:54:17 Snapshots are currently experimental. File formats may be subject to change.
2016-08-23 18:54:17 Taking snapshot starting at block 2125024
2016-08-23 18:54:22 Snapshot: 0 accounts 30000 blocks 45267752 bytes
2016-08-23 18:54:27 Snapshot: 26301 accounts 30000 blocks 48779875 bytes
2016-08-23 18:54:32 Snapshot: No progress since last update.
2016-08-23 18:54:37 Snapshot: 42547 accounts 30000 blocks 55712956 bytes
2016-08-23 18:54:42 Snapshot: 93501 accounts 30000 blocks 62398616 bytes
2016-08-23 18:54:47 Snapshot: 113748 accounts 30000 blocks 66731907 bytes
2016-08-23 18:54:52 Snapshot: 150226 accounts 30000 blocks 71952385 bytes
2016-08-23 18:54:57 Snapshot: No progress since last update.
2016-08-23 18:55:02 Snapshot: 162903 accounts 30000 blocks 78055364 bytes
2016-08-23 18:55:07 Snapshot: 178192 accounts 30000 blocks 86649930 bytes
2016-08-23 18:55:12 Snapshot: 185832 accounts 30000 blocks 89311494 bytes
2016-08-23 18:55:17 Snapshot: 187906 accounts 30000 blocks 97146953 bytes
2016-08-23 18:55:22 Snapshot: 203503 accounts 30000 blocks 102396633 bytes
2016-08-23 18:55:27 Snapshot: 241343 accounts 30000 blocks 107764603 bytes
2016-08-23 18:55:32 Snapshot: 278393 accounts 30000 blocks 112910349 bytes
2016-08-23 18:55:37 Snapshot: 321436 accounts 30000 blocks 120088068 bytes
2016-08-23 18:55:42 Snapshot: 374931 accounts 30000 blocks 126544579 bytes
2016-08-23 18:55:47 Snapshot: 438118 accounts 30000 blocks 135104792 bytes
2016-08-23 18:55:52 Snapshot: 461411 accounts 30000 blocks 139010897 bytes
2016-08-23 18:55:57 Snapshot: No progress since last update.
2016-08-23 18:56:02 Snapshot: No progress since last update.
2016-08-23 18:56:07 Snapshot: 500755 accounts 30000 blocks 153717847 bytes
2016-08-23 18:56:12 Snapshot: 539426 accounts 30000 blocks 161693878 bytes
2016-08-23 18:56:13 produced 57 state chunks and 29 block chunks.
2016-08-23 18:56:13 snapshot creation complete
$ du -h ~/1.3.0.snap 
156M    /home/robert/1.3.0.snap

new:

$ parity snapshot ~/1.4.0.snap
2016-08-23 18:57:13 Configured for Frontier/Homestead using Ethash engine
2016-08-23 18:57:26 Snapshots are currently experimental. File formats may be subject to change.
2016-08-23 18:57:26 Taking snapshot starting at block 2125024
2016-08-23 18:57:31 Snapshot: 0 accounts 18979 blocks 29242458 bytes
2016-08-23 18:57:36 Snapshot: 29053 accounts 30000 blocks 47497159 bytes
2016-08-23 18:57:41 Snapshot: 91676 accounts 30000 blocks 57638375 bytes
2016-08-23 18:57:46 Snapshot: 152458 accounts 30000 blocks 69295380 bytes
2016-08-23 18:57:51 Snapshot: 178192 accounts 30000 blocks 77891035 bytes
2016-08-23 18:57:56 Snapshot: 269134 accounts 30000 blocks 98322683 bytes
2016-08-23 18:58:01 Snapshot: 395761 accounts 30000 blocks 109582231 bytes
2016-08-23 18:58:06 Snapshot: 460830 accounts 30000 blocks 114707581 bytes
2016-08-23 18:58:11 produced 31 state chunks and 28 block chunks.
2016-08-23 18:58:12 snapshot creation complete
$ du -h ~/1.4.0.snap 
130M    /home/robert/1.4.0.snap

Unfortunately breaks backwards compatibility, but that's why snapshots were marked as experimental.

Fixes #1981

[coveralls]

Changes Unknown when pulling de8853e on snapshot_optimizations into * on master*.

[coveralls]

Coverage decreased (-0.06%) to 86.702% when pulling c1b0c1b on snapshot_optimizations into f6b8dd0 on master.

[rphmeier]

(snapshot coverage is really bad right now, I'm working on it but proper integration tests should really be their own PR)