should cancel existing archive.open when reset is cleared

[clkao]

otherwise the previous archive can pop in after the user clicked reset

[okdistribute]

how do you reproduce this?

[clkao]

1. in tab1, open dat.land and add some files
2. in tab2, use the link in tab1 and wait until the files appear
3. reload tab1, click "Reset" when the message is still 'initializing'
4. tab1 will eventually show the original content, replicated from tab2, discarding the fresh archive created by user just now