server

- Changed replaying to always allocate a separate THD object
  for applying log events. This is to avoid tampering original
  THD state during replay process.
- Return success from sp_instr_stmt::exec_core() if replaying
  succeeds.
- Do not push warnings/errors into diagnostics area if the
  transaction must be replayed. This is to avoid reporting
  transient errors to the client.

Added two tests galera_sp_bf_abort, galera_sp_insert_parallel.
Wsrep-lib position updated.