Truncated DLLs cause suspended queries

Query .so's that have been partially copied cause suspended queries that can only be recovered by manual deletion of the query dll (to allow it to re-copy from another location).

Such truncated .so files can occur if Roxie is restarted while in the middle of copying a dll, of if query dlls being synced manually are interrupted.

We can make the dllserver code more robust - copy to temporary then rename (so less likely to cause truncated queries) and also try fetching again if we get an error on the local dll.