storage_proxy: mutate_atomically fix for consistency and BL removal
storage_proxy: mutate_atomically fix for consistency and BL removal

The change to use consistency_level::ONE in send_batchlog_mutation
sort of fixes #478, but is not 100% correct.
When doing async_remove_from_batchlog, the CL is actually supposed to
be ANY.

Also, we should _not_ remove the batch log mutation from any nodes
if the mutate fails, since having it there in case of failure is sort of
the whole point of it. I.e. async_remove_from_batchlog should not be
called from a "finally", but from a "then".

Refs #478

kjniemi/scylla,kjniemi/scylla,raphaelsc/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla,kjniemi/scylla,scylladb/scylla,avikivity/scylla,duarten/scylla,scylladb/scylla,raphaelsc/scylla,raphaelsc/scylla,avikivity/scylla,avikivity/scylla,duarten/scylla