Speed up Sakia by computing average timestamps

src/sakia/data/entities/transaction.py

src/sakia/data/processors/blockchain.py

src/sakia/data/processors/dividends.py

src/sakia/data/processors/transactions.py

src/sakia/gui/dialogs/connection_cfg/controller.py

src/sakia/gui/dialogs/connection_cfg/model.py

src/sakia/gui/navigation/graphs/wot/controller.py

src/sakia/services/blockchain.py

src/sakia/services/identities.py

src/sakia/services/network.py