refactor db_upgrader, more meaningful for dst_db_path parameter

[HarrisChu]

in db_upgrader, when we upgrade from v1 to v2:

1. read from src_db_path, and then write to dst_db_path

but when we upgrade from v2 to v3:

1. rewrite to src_db_path directly.
2. dst_db_path is useless.

so, there're some issues:

1. dst_db_path would do nothing, but still copy wal to dst, so need to create a directory.
2. won't copy data to dst_db_path, a little confused.
3. there is no effective for --compactions parameter.

[HarrisChu]

BTW, could we make the checkpoint automatically?
and then no need for users to backup the original data manually.

cc @randomJoe211