tests: stress: remove obsolete usleep()
XXX: api: simplified prepare/plain/commit

1) Prepare now do not accept wc argument only to extract size from it.
2) Plain write now does not result in commit
3) Commit now does not accept wc, but also only needed size and flags.
XXX: api: unified API
config: removed bsize and cache_size config options

Also added different types of padding to eblob_config structure. From
now on it'll be easier to add new fields to it.