Refactored wsrep patch to not use LOCK_thread_count and COND_thread_count anymore.
This has partially been replaced by using old LOCK_wsrep_slave_threads mutex.
For slave thread count change waiting, new COND_wsrep_slave_threads signal has been added
Added LOCK_wsrep_cluster_config mutex to control that cluster address change cannot happen in parallel
Protected wsrep_slave_threads variable changes with LOCK_cluster_config mutex
This is for avoiding concurrent slave thread count and cluster joining operations to happen
Fixes according to Teemu's review
* 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.
