backtracegenerator: implement a serialization lock

to prevent systems from drowning in gdb instances, we now establish a serialization lock in the form of a file lock. only a single debugger may be active at a time

this only partially fixes the problem since a single debugger may still OOM the system. we still want a better solution, such as resource constraining via systemd, in the long run.

It's a bit garbage to test.

systemctl --user stop drkonqi-sentry-postman.timer drkonqi-sentry-postman.path
for f in 1 2 3 4; do kwrite& done
sleep 4;
killall -SEGV kwrite

then make sure ~/.cache/drkonqi/sentry-envelopes/ contains 4 envelopes and delete them lest they get sent to sentry XD

CCBUG: 489315