Keep session state across hydra-server restarts

Dependencies

• [2] VBUNNYXG Fix path to the session data
• [3] HSVVEKTY * Start of a JSON API to get information about a specific build.
• [4] PCKLFRT5 Support push notification of repository changes
• [*] J5UVLXOK * Start of a basic Catalyst web interface.

Change contents

• replacement in src/lib/Hydra.pm at line 57
  [3.649]→[2.0:68](∅→∅)
  − storage => Hydra::Model::DB::getHydraPath . "/session_data"
  [3.649]

  [3.649]
  + storage => Hydra::Model::DB::getHydraPath . "/session_data",

  + unlink_on_exit => 0