Before we introduced objects versions internally move operation was
always failing when under target location object exists. But then we
had only single version 1 all the time. With versions different than 1
we need to check all existing objects under target location.
To be backward compatible with our API new logic looks like this:
* if there is no object under target location use source object version
as target version
* if there are only pending objects find first free (highest) version
which could be used to move object there
* if there is committed object under target location reject move
operation
Fixes https://github.com/storj/storj/issues/5403
Change-Id: I717f3e7c42470b406287d6ec335f6f057d3fc3b5
77afdae741