Fix reading of .osc.gz, use 'zcat' command.
Merge branch 'master' into ser
Set visible flag in ser/handler and show in debug output
Ignore object versions coming by we already have.

When reading a planet dump and later a change files it can happen that
we already have a version of an object that appears in the change file.
In that case we ignore that object.
Add remove() method to map.
Handle object deletion.

Unfortunately deleted objects have the same version as the last existing
version of an object. So we specifically have to handle this case.
Add some misc functions