* Removed all references related to wsrep_thd_pool (which was removed)

* Removed unused declarations in wsrep_schema.h

* The following would result invalid reads in
  Wsrep_schema::replay_transaction():
  ```
  frag_table->field[4]->val_str(&buf);

  Wsrep_schema_impl::end_index_scan(frag_table);
  Wsrep_schema_impl::finish_stmt(thd);
  ret= wsrep_apply_events(thd, rli, buf.c_ptr_safe(), buf.length());
  ```

  because `buf` was accessed after closing the table. The fix is to
  perform storage reads using a different THD.

* In Wsrep_schema::recover_sr_transactions(), cluster_table was opened
  for write, however it is only read here. And frag_table was opened
  for read, wereas write is potentially needed.
  Also, avoid copy caused by String::c_ptr() to zero terminate the c
  string, use c_ptr_quick instead.