!fix/feat(epochs): fix start/end times, introduce block times and active_stake

[1000101]

Epochs improvements and fixes.

Improvements:

• introduced first_block_time and last_block_time which are first and last block times of the queried epoch
• introduced active_stake which is the sum of amount staked in queried epoch

Fixes:

• start_time and end_time times will correctly reflect epoch times (previously, first and last block times were displayed, respectively)

! Breaking change - unify terminology.

• blocks_count -> block_count
• txs_count -> tx_count
• txs_sum -> output
• fees_sum -> fees

Affected endpoints: /epochs/latest, /epochs/{number}, /epochs/{number}/next, /epochs/{number}/previous.

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