Remove checks for keys and buckets during transfers from verify_listkeys

There is no way to ensure what portion of the full harvest is returned by a
coverage request during handoffs. A node may start accepting requests before
the data has been handed off and there is no way to prevent or detect this and
no way to have a deterministic test condition that checks for the presence of
all expected keys. This change removes any checks during transfers. It also
and adds a directive to wait until transfers have completed after a node is
added or removed and before listkeys result verification resumes.