Checksum precalculation

[sparkoo]

Calculating box checksums on demand has too high performance cost for http request (~1.5GB box takes about 5s to calculate md5 checksum). So we need to calculate checksums on background, persist results and on request just read those checksums.

• checksum calculation must not block any request -> must run on background
• BackgroundChecksumCalculator calculates checksums with given configuration for all boxes and persist them with HashStore
• BackgroundChecksumCalculator must keep checksums in latest state -> if new box is added when Boxitory already running, it must catch it and calculate checksum
• once checksum is calculated and persisted, it can't be replaced
• new configuration option box.checksum_precalculate
  • boolean value - true|false
  • tells whether checksums for boxes should be precalculated on background
  • default true
• probably there will be some other (advanced) configuration option configuring background calculation
• probably BackgroundChecksumCalculator interface with FilesystemBackgroundChecksumCalculator implementation
• Don't forget to test and keep it as independent as possible. Again, there must be space for replace storing checksums to file by e.g. store in database.

[sparkoo]

depends on #24

[sparkoo]

postponed, as current behavior seems sufficient