WL#5905: PROCESSLIST: show thread type

==== Goal ====

The goal of this worklog is to add one column to SHOW PROCESSLIST and to 
INFORMATION_SCHEMA.PROCESSLIST. The column should display the thread type and have 
the SQL type:

  ENUM('Client',
       'Slave SQL',
       'Slave IO',
       'Insert delayed',
       'NDB binlog',
       'Event scheduler',
       'Event worker');

==== Background ====

It is usually possible for a human to guess the thread type when reading the 
output from SHOW PROCESSLIST, but it is hard to compute it programmatically, e.g., 
from a script or test case.