WL#7729: Instrumentation of connection type

MySQL lacks DBA visibility into the types of connections being established and
used; there is no meaningful way to distinguish SSL connections from TCP/IP
connections, or socket, shared memory or named pipe connections.  This causes
operational difficulty in assessing security.  Instrumentation should include:

1.  Connection type information in P_S.THREADS
2.  Connection type information in audit log
3.  Connection type information in general log

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

http://dev.mysql.com/doc/relnotes/mysql/5.7/en/news-5-7-8.html
http://dev.mysql.com/doc/refman/5.7/en/threads-table.html
http://dev.mysql.com/doc/refman/5.7/en/query-log.html
http://dev.mysql.com/doc/refman/5.7/en/writing-audit-plugins.html

FR 1 : Connection types will be displayed consistently in all logging and 
diagnostic output:
FR 1.1 : Plain TCP/IP connections (VIO_TYPE_TCPIP) will be displayed as "TCP/IP"
FR 1.2 : Socket connections (VIO_TYPE_SOCKET) will be displayed as "Socket"
FR 1.3 : Named pipe connections (VIO_TYPE_NAMEDPIPE) will be displayed as
"Named Pipe"
FR 1.4 : TCP/IP connections over SSL/TLS (VIO_TYPE_SSL) will be displayed as
"SSL/TLS"
FR 1.5 : Shared memory connections (VIO_TYPE_SHARED_MEMORY) will be displayed
as "Shared Memory"

FR 2 : PERFORMANCE_SCHEMA.THREADS table will add a new column to display
connection type for each thread
FR 2.1 : Column name will be "CONNECTION_TYPE"
FR 2.2 : Column type will be VARCHAR(16)
FR 2.3 : Internal threads will show as NULL

FR 3 : Audit log connection events will display connection type connection event 
types
FR 3.1 : Connection type information will be added in a new XML element named 
"CONNECTION_TYPE"
FR 3.2 : QUIT event types will have an empty CONNECTION_TYPE element

FR 4 : Connection type information will be reflected in general query log entries 
for new connections
FR 4.1 : Existing general query log entries for connections will be extended with 
" using [connection type]" 

MySQL Server supports various connection types (TCP/IP, SSH/TLS, socket, shared
memory and named pipe), but information about the type of connection is not
exposed to DBAs via standard interfaces.  This information is needed in the
following interfaces:

* PERFORMANCE_SCHEMA.THREADS table
* Audit log interface
* Audit log file
* General query log

All output representation of connection type information should be consistently
expressed.

Upgrades from earlier versions of MySQL Server having PERFORMANCE_SCHEMA.THREADS
table definition lacking the new column will be supported.

Documentation will be updated to reflect changes to the audit log connection
event structure.

The following modules will be modified:

sql/auth/sql_authentication.cc :
* Add lookup array for mapping enum_vio_type to strings
* Add call to set P_S instrumentation value for connection type
* Add connection type information to general query log for connection events

sql/sql_audit.h :
* Add connection type information to connection event notifications

sql/sql_audit.cc : 
* Add connection type information to class handler

include/msyql/plugin_audit.h :
* Add connection type information to mysql_event_connection struct

internal/plugin/audit_log/audit_log.cc :
* Put connection type in connection type event logging

storage/perfschema/pfs.cc :
* Extend PSI_v1 with method for setting connection type

storage/perfschema/table_threads.cc:
* Extend field types to include CONNECTION_TYPE
* Add connection type information to make_row()
* Add connection type information to read_row_values()
* Add CONNECTION_TYPE to exclusion list in update_row_values()

storage/perfschema/table_threads.h:
* Add connection type to row_threads struct

scripts/mysql_system_tables.sql : 
* Modify THREADS table structure to include CONNECTION_TYPE column

include/mysql/psi/psi.h : 
* Define method to set connection type in P_S instrumentation
* Define type for connection type method

include/mysql/pfs_thread_provider.h : 
* Define method to set connection type in P_S instrumentation