buffered logs & log level

[pythcoiner]

This PR add one new field to ElectrsD:

• logs: an mpsc::Receiver<String> into wich the electrs logs are redirected if Conf.buffered_logs is set to true.

and add 2 new fields to the Conf struct:

• log_level: specify the log level to be passed to electrs (log::Level::Info as default)
• buffered_logs: option to enable redirecting electrs stdout/stderr to ElectrsD.logs.

Note: the log crate is also reexported in order the consumer can use log::Level enum

[pythcoiner]

Note: args for passing the log level seems to be different ont mempool implementation (maybe also esplora?), i need figure it out