WL#8853: PERFORMANCE_SCHEMA, ADD COLUMN THREADS.THREAD_OS_ID

Add a column THREAD_OS_ID to table performance_schema.threads

Column DDL:
-----------

`THREAD_OS_ID` bigint(20) unsigned DEFAULT NULL

Semantic:
---------

Expose the "TID", or thread/task identifier, as defined by the underlying
operating system.

For example with Linux, this corresponds to gettid().

TID is a field exposed in particular:
- in perf: http://man7.org/linux/man-pages/man1/perf-stat.1.html
- is ps -L: http://man7.org/linux/man-pages/man1/ps.1.html
- in /proc/[pid]/task/[tid]: http://man7.org/linux/man-pages/man5/proc.5.html

For Windows, the thread id is visible in Process Explorer:
https://technet.microsoft.com/en-us/sysinternals/bb896653.aspx

Benefits:
---------

By exposing the thread TID in the performance schema, a dba can correlate:
- a user session (see existing column PROCESSLIST_ID)
- a server thread (see existing column THREAD_ID)
- to an operating system Light Weigh Process (LWP), aka a thread, with new
column THREAD_OS_ID.

This is critical to inspect / monitor MySQL server threads with tools external
to the server itself, such as linux perf.

User Documentation
==================

* http://dev.mysql.com/doc/relnotes/mysql/5.7/en/news-5-7-9.html
* http://dev.mysql.com/doc/refman/5.7/en/threads-table.html

Requirements
============

(1) Table performance_schema.threads.
-------------------------------------

Func-Req (1.1): A fresh MySQL installation of CURRENT-VERSION must create the
table performance_schema.threads with the new THREAD_OS_ID column.

Func-Req (1.2): An upgrade from PREVIOUS-VERSION to CURRENT-VERSION must create
the table performance_schema.threads with the new THREAD_OS_ID column.

Func-Req (1.3): Column threads.THREAD_OS_ID is read-only, by users with the
proper grants.

Func-Req (1.4): When a MySQL thread is associated to the same operating system
thread for its lifetime, column THREAD_OS_ID contains the operating system
thread id.

Func-Req (1.5): When a MySQL thread is not associated to the same operating
system thread for its lifetime, column THREAD_OS_ID contains NULL.

Note: this typically happens for the thread_pool, as a user session is not
permanently associated to a worker thread, to the THREAD_OS_ID column will be
NULL for user sessions.