Management API: change log level on the fly

[folex]

Ability to change node's log level and log filters without restarting the node. That would simplify debug process by node operators while keeping the network more stable.

Technical details

• RUST_LOG env variable currently manages log level from the outside
• env_logger is used to control logging
• WASM_LOG env variable controls log level of the underlying Wasm Runtime (FCE)

Things to consider

• tracing library may provide more convenient API for the task while also enabling more structured logging. Worth looking into, but not necessary.
• Most likely, to change FCE log level one would need to add special API to FCE as it is most likely isn't possible to change WASM_LOG without recreating FCE