Prevent running multiple instances

[maximbaz]

Right now running multiple instances of scli will not work anyway (one of them will be locked on signal-cli), and by accidentally doing this I somehow managed to wipe a few days of history (opened second instance, realized mistake and closed but closed both).

We could perhaps just add some code to prevent history overwrite until we get a lock from signal-cli, but maybe there is more we could do, have some sort of our own lock, and quicker abort if it's a second instance?

[exquo]

Yes, this should be address. Thanks for raising it!

Multiple instances probably shouldn't be allowed even if both run without --save-history, as they can still call signal-cli's dbus methods (even though only one signal-cli's daemon instance will be running), showing different chat histories, and perhaps other unpredictable behavior.

Multiple instances should be allowed, however, if they use different --usernames (accounts / phone numbers). But #172 will need to be fixed first.