loader: move uploaded view pending sstables to staging
loader: move uploaded view pending sstables to staging

When loading tables uploaded via `nodetool refresh`, they used to be
left in upload/ directory if view updates would need to be generated
from them. Since view update generation is asynchronous, sstables
left in the directory could erroneously get overwritten by the user,
who decides to upload another batch of sstables and some of the names
collided.
To remedy this, uploaded sstables that need view updates are moved
to staging/ directory with a unique generation number, where they
await view update generation.

Fixes #4047

scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla