Rewrite state and stats serialization

Currently serialization is based on three types of files per task:

• base task data
• rarely changing state
• often changing state.

Issues we have:

• whole files are re-written over and over again,
• code is ugly in a way that for some objects two types of serialization needs to be supported,
• there are often problems with corrupted state files.

Rewrite the serialization mechanism to use sqlite database as an underlying engine (with possibility to switch it to something else if needed).