before this change, we failed to apply the filtering of tablestats
command in the right way:
1. `table_filter` failed to check if delimiter is npos before
extract the cf component from the specified table name.
2. the stats should not included the keyspace which are not
included by the filter.
3. the total number of tables in the stats report should contain
all tables no matter they are filtered or not.
in this change, all the problems above are addressed. and the tests
are updated to cover these use cases.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closesscylladb/scylladb#17468
