Add 'no-blockchain' blockchain_source option.

[AdamISZ]

This is intended primarily to support running the
ob-watcher.py script from any environment that has no
blockchain access.
It could be used in debug and possibly recovery scenarios,
but this is explicitly unsupported (see comments in config).

[kristapsk]

Makes sense, ACK 931a5c8 (tested with no Bitcoin Core running)

[chris-belcher]

Could this also work for exporting individual private keys using the -H option of wallet-tool ?

Edit: By my brief reading of the code it might work for methods in the noscan_methods list in wallet_tool_main()

[AdamISZ]

Yeah we could think this through a bit, sorry I was being a bit lazy :) Could be used for noscan methods right?

I think where it starts to get tricky is when we want to support "loading" a wallet without any usage information. There starts to be a danger of misleading users.
But for stuff where they're expert users and/or they only want some very specific information, it could be good. Obvious ones would be getting the seedphrase for recovery and getting individual keys (or batches of them). Or just printing out lists of addresses.

[AdamISZ]

Btw was also thinking about going further and supporting display of some information on the GUI with this, by adding some kind of connectivity icon in the status bar that would show clearly a 'disconnected' state, but it's a significantly more complicated thing. Perhaps in future.

(If we did this it could make some sense to have a connectivity heartbeat loop in the wallet service or blockchain code so you could actually show if the rpc connection to Core was unavailable, also. On CLI the same function could just have the effect of an error level logging message.)

[kristapsk]

Accidentally had kept this setting from previous testing and noticed that joinmarket-qt will fail on wallet load with error, but on console output only, no messages in GUI itself. Probably it's worth adding check there, that it will not even start with a such configuration (with an error in GUI message box).

2020-01-03 21:37:35,840 [ERROR]  Failure to get blockheight from Bitcoin Core:
2020-01-03 21:37:35,840 [ERROR]  AttributeError("'NoneType' object has no attribute 'rpc'",)

[kristapsk]

Of course, not if the some partial support of this is added to the GUI, but likely it will remain in "perhaps in future" state for at least some time. :)

[AdamISZ]

Yeah this needs to have something in GUI so the user knows what happened, and corrects the setting, will do, thanks.

[AdamISZ]

8ae41b1 tests OK (error message box and quit if no-blockchain configured with Qt).