Shard 0 can clear the in-progress build status of all shards when a
view finishes building, because we are ensured all writes to the
system table have completed with earlier timestamps.
This is not the case when dropping a view. A drop can happen
concurrently with the build, in which case shard 0 may process the
notification before another shard receives it, and before that shard
writes to the system table.
Fix this by ensuring each shard clears its own status on drop.
Signed-off-by: Duarte Nunes <duarte@scylladb.com>
e683c1367f