improve performance during long unfinality periods

[pk910]

follow up PR on #223 with more performance improvements and fixes for long unfinality periods as currently seen on devnet-5:

• improve GetEpochStats performance (avoid expensive chain traversal)
• improve GetCanonicalHead performance (sort head candidates once only)
• store proposer duties to db to avoid expensive recalculation on startup
• fix SQL issue when finalizing a epoch without canonical blocks (ERROR: syntax error at or near \")\" (SQLSTATE 42601))
• fixed updating the fork_id for canonical blocks in the finalization handler