implement tracker-wide garbage collector

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
allow to change the GC parameters at runtime

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
make gc params for net latency module configurable at load time

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
doc

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Call the callback even when overriding non-unique keys (if enabled)

Allows the user to track the overrides if necessary.

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>