fio: Introduce new constant thinkcycles option

fio: Introduce new constant thinkcycles option

The thinkcycles parameter allows to set a number of cycles to spin between
requests to model real-world applications more realistically

The thinktime parameter family can be used to model an application processing
the data to be able to model real-world applications more closely.
Unfortunately this is currently set per constant time and therefore is affected
by CPU frequency settings or task migration to a CPU with different capacity.
The new thinkcycles parameter closes that gap and allows specifying a constant
number of cycles instead, such that CPU capacity is taken into account.

Signed-off-by: Christian Loehle <christian.loehle@arm.com>