sync_schema is supposed to make sure that this node knows about all
schema changes known by "nodes" that were made prior to this call.
Currently, when a node is down, the sync is sliently skipped.
To fix, add a flag to migration_task::run_may_throw to indicate that it
should fail if a node is down.
Fixes#4791
All database code was converted to is when storage_proxy was made
distributed, but then new code was written to use storage_proxy& again.
Passing distributed<> object is safer since it can be passed between
shards safely. There was a patch to fix one such case yesterday, I found
one more while converting.
