Three host functions used by the runtime and not in the specs

[tomaka]

The three following host functions are imported by the Polkadot/Westend runtimes, but aren't documented on https://spec.polkadot.network/:

• ext_offchain_index_set_version_1
• ext_offchain_index_clear_version_1
• ext_logging_max_level_version_1

[bhargavbh]

addressed in the PR-642