MB-12591 Provide a doc count for both file and KV store levels

- Now doc count and space estimation are provided for each KV store
separately.
- Such statistics are globally managed so that the legacy 'ndocs' and
'datasize' attributes in the fdb_kvs_handle are removed.
- fdb_get_kvs_info returns the number of docs belonging to the KV store,
while fdb_get_file_info returns the total number of docs in the file.