* Adds method wsrep::transaction::streaming_step() so that there is a
single place where streaming context unit counter is udpated.
The method also checks that some data has been generated before
attempting fragment replication.
* Emit a warning if there is an attempt to replicate a fragment and
there is no data to replicate.
* Count separately fragments certified and fragments stored in
streaming context. Storing the fragment may ultimately fail
due to BF abort even if the fragment was succesfully certified.
Therefore we need to have separate counter for certified fragments
to determine if the transaction is streaming and seqnos of fragments
which have been succesfully stored.
* Provider release is called only after succesful fragment certification
and fragment store.
* Fixed handling of write sets with rollback flag set in apply_write_set()
* Added server_id into transaction in order to be able to stop
streaming applier during high priority BF abort
* Added missing commit fragment applying
* Don't clear fragments for replaying SR transaction