Simplify batch API methods

Now that we've moved away from the older batch impl. based on the go version (saving the batch bsos into a json blob), we can simplify a couple parts of the batch API:

• Remove Db::batch_get: This method was somewhat useful for the old version (mostly for tests). It's similar to validate_batch so at this point it seems unnecessary. commit_batch should take a batch_id instead of a params::Batch. Additionally validate_batch is called from append (and elsewhere in MySql). So a batch post call is probably calling get/validate 3 times for no good reason.

• Remove the bsos argument (and the do_append work) from create_batch. It's not needed: the batch handler always specifies a bsos value of vec![]

• results::CommitBatch is results::PostBsos (as it used to return the result of it). It should be a SyncTimestamp instead as the other fields of PostBsos aren't used here #934

And some associated tests will need to reflect these changes too

Start by tracing Db::batch_get, commit_batch, validate_batch, append, and create_batch, then inspect the associated tests mentioned in the issue. Identify all callers and update the batch API consistently: remove Db::batch_get, pass batch_id to commit_batch, remove the bsos argument and do_append work from create_batch, and make the tests pass.