An assertion

  `server_state_.rollback_mode() == wsrep::server_state::rm_async`

fired in before_command() when
- thread-handling was set to pool-of-threads and
- a BF abort happened between client session calls to
  wait_rollback_complete_and_acquire_ownership() and before_command().

This commit introduces a test case to reproduce the crash and
updates wsrep-lib submodule to fixed version.