Soft prevention of concurrent bots with 1 wallet using a lock

[AdamISZ]

Idea iirc due to nkuttler (although let's face it, it should be pretty obvious!):

Include some kind of .lock file to prevent more than one process succeeding in accessing a single joinmarket wallet. It's not that this is necessarily dangerous to funds (given the nature of coinjoin), but the code was never written with this being possible in mind, and I for one have no desire to do the work necessary to support it. Also of course dissuading it has a relationship to JoinMarket-Org/joinmarket#693 .

I could envisage doing this at the same time as #62 (comment)

Also, it's very simple, if anyone else feels inclined to do it, go ahead.

[undeath]

implemented with #104