Support for detaching prepared XA transactions

Add support for detaching XA transactions. This is useful for handling
the case where the DBMS client has a transaction in prepared state and
disconnects. Before disconnect, the DBMS calls the newly introduced
client_state::xa_detach(), to cleanup the local transaction and
convert it to a high priority transaction. The DBMS may later attempt
to terminate the transaction through client_state::commit_by_xid() or
client_state::rollback_by_xid().

Also in this patch:

- Fix client_state::close() so that it does not rollback transactions
  in prepared state
- Changed class wsrep::xid representation to hold enough information
  so that DBMS can convert to its native representation
- Fix potential infinite loop in
  server_state::find_streaming_applier(wsrep:xid&)
- Append SR keys on prepare fragment and make it pa_unsafe
- Handle one phase commit (simply fall back to two phase)
- Do not rollback prepared streaming clients in
  server_state::close_orphaned_transactions()

include/wsrep/buffer.hpp

@@ -66,6 +66,10 @@ namespace wsrep
             : buffer_()
         { }
 
+        mutable_buffer(const mutable_buffer& b)
+            : buffer_(b.buffer_)
+        { }
+
         void resize(size_t s) { buffer_.resize(s); }
 
         void clear()
@@ -111,6 +115,11 @@ namespace wsrep
             buffer_ = other.buffer_;
             return *this;
         }
+
+        bool operator==(const mutable_buffer& other) const
+        {
+          return buffer_ == other.buffer_;
+        }
     private:
         std::vector<char> buffer_;
     };