Global thread id

Currently, mxs send handshake to client,so the client get the thread id produced by mxs, after the authentication between client and mxs is finished, mxs connect to backend, and backend produce another thread id, these two thread ids are not the same, it will be lead to some problem;

Proposal:
1. use mariadb's sequence mechanism to get a thread id range（https://mariadb.com/kb/en/library/sequence-overview），and mxs generate thread id in the range;
2. when connect to the backend, put the thead id in the handshake response's attribute filed,and server use the given thread id;