v0.2.6: support $_field_primary_key, and $_enable_delete_all

[vmico]

1. better readme and note for most common CRUD apis.
2. better #softDeletion and #softRecovery with $_field_flag_hidden
3. avoid to delete all rows with $_enable_delete_all
4. better performance to countAsync with $_field_primary_key
5. [API-D] remove api: DbTable.findLimitAsync()
6. [API-A] add api: DbTable.getOrNullAsync()