folder_block_manager: fix deadlock while archiving during deletes
folder_block_manager: fix deadlock while archiving during deletes

If the archiveChan buffer is full, blocking on a send on it while
processing deletes leads to deadlock.  Blah.

Issue: KBFS-777

keybase/kbfs,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client