sstables/metadata_collector: move the default values to the global tracker
sstables/metadata_collector: move the default values to the global tracker

column_stats is a per-partition tracker, while metadata_collector is the
global one. The statistics gathered by column_stats are merged into the
metadata_collector. In order to ensure that we get proper default values
in case no value of particular kind (e.g. no TTLs) was seen they need to
be set on the global tracker, not the per-partition one.

avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla