[mysql] Don't report db failures from `post_bsos`

[pjenvey]

This involves a couple parts:

1. MysqlDb should match both the Spanner backend and the Python behavior here: any errors from db calls should not be reported as part of the "failed" bucket, they should propagate up instead as normal.
2. After 1., the Db layer won't utilize failed buckets at all. So we should cleanup both params::PostBsos and results::PostBsos to no longer include failed. In fact result::PostBsos doesn't need a success bucket either: kill the whole struct and just return a SyncTimestamp from post_bsos

This contributes partly to the effort needed for #418.