cpimport: simultaneous calls for the same table fail becuase of lock

If two cpimport jobs are submitted closely such that they both run together and they are for the same single table, then there's a chance both will fail because the lock system may have a bug.

The theory is that cpimport without a -P <pm list> option tries to lock the table without a dbroot list. TableLockServer::lock() doesn't find a lock that already exists because there's no dbroot overlap.

Recommend we modify TableLockInfo::overlaps() – used by lock() – to return true if the dbrootlist is empty().