[#433] Add approximate block interval getter

Description

Problem: Sometimes logic of the contract may rely on the block intervals. We'd like to be able to test such contract on the blockchains with different block intervals (e.g. fast local-chain and slow testnet).

Solution: Add 'getApproximateBlockInterval' getter which returns the minimal possible delay between two subsequent blocks.

Related issue(s)

Resolves #433 (closed)

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