tracing: Generalize hist trigger onmax and save action

The action refactor code allowed actions and handlers to be separated,
but the existing onmax handler and save action code is still not
flexible enough to handle arbitrary coupling.  This change generalizes
them and in the process makes additional handlers and actions easier
to implement.

The onmax action can be broken up and thought of as two separate
components - a variable to be tracked (the parameter given to the
onmax($var_to_track) function) and an invisible variable created to
save the ongoing result of doing something with that variable, such as
saving the max value of that variable so far seen.

Separating it out like this and renaming it appropriately allows us to
use the same code for similar tracking functions such as
onchange($var_to_track), which would just track the last value seen
rather than the max seen so far, which is useful in some situations.

Additionally, because different handlers and actions may want to save
and access data differently e.g. save and retrieve tracking values as
local variables vs something more global, save_val() and get_val()
interface functions are introduced and max-specific implementations
are used instead.

The same goes for the code that checks whether a maximum has been hit
- a generic check_val() interface and max-checking implementation is
used instead, which allows future patches to make use of he same code
using their own implemetations of similar functionality.

Link: http://lkml.kernel.org/r/980ea73dd8e3f36db3d646f99652f8fed42b77d4.1550100284.git.tom.zanussi@linux.intel.com

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>