Return last_unused addresses, parse .ini and create online wallet at startup

[notmandatory]

Changed:

• /bitcoin returns last unused address instead of a new address every time, fixes New unused addresses #7
• /bitcoin/api/new renamed to /bitcoin/api/last_unused which also returns last unused address
• load and parse .ini file prior to starting server
• create wallet once prior to starting server and share between requests
• create wallet as online to be able to monitor what the last unused address is
• specify the electrum server url in .ini file

I also ran cargo fmt, if everyone uses it it will be easier to review and merge :-)

[notmandatory]

BTW, this turned into a bit larger change than I originally intended so happy to discuss any parts of it and how to integrate with what you're plans are for this project.

[lvaccaro]

ack 6983ce4

Thanks to fix other cosmetic stuff in the code.
I think a user could always put global vars into a file and run source to load the content.

Maybe it is time to brainstorming ideas in a separate issue: #14

[notmandatory]

I'm going to redo this PR to have just the last_unused part and not the struct_opt stuff. I'll create a new PR for that if it makes sense later.

[notmandatory]

I'm done scaling back this PR to the things mentioned in the description.

I'd also like like to update the CI to make sure cargo fmt is always ok, but can do that separately if it works for you.

[lvaccaro]

utack 4f2e6a6