Improve throughput performance (trigger local subscriber focus)

[jean-roland]

There is a small performance dip in non-fragment peer throughput compared with 0.11. Profiling helped identify a much longer time spent in trigger_local_subscriberand this PR aims to fix this.

• Static inline functions: Functions that should have been inlined by the compiler (like z_sample_null) weren't. This is solved by explicitly setting all the _null, _check and _alias functions to static inline.

• Null functions: A lot of time was spent initializing types to 0 compared to just using memset(0), switching all the _null functions to a {0} initialization closed this gap and is less prone to error.

• Slice init / string preallocate: This functions had many superfluous checks / init. They got simplified.

• get_expanded_key_by_key: Added a shorter path for already expanded keys.

• trigger_local_subcriber: Added a shorter path when there is 0 subscribers to trigger. Added a config token, off by default, to execute this function.

The fastest code is the one we don't execute

[github-actions[bot]]

PR missing one of the required labels: {'internal', 'enhancement', 'dependencies', 'new feature', 'documentation', 'breaking-change', 'bug'}

[DenisBiryukov91]

Looks good, but I do not really like the idea of memsetting everything to 0, especially for dependent structs. In case we modify null-constructors for their fields, we risk to face some difficult to detect bugs. I'm wondering if memsetting to 0 really provides a speed boost that is worth the cost.

[jean-roland]

I understand the issue, but then maybe we should make a distinction between _null which would be some kind of memory initialization and _init that would be a type initialization, in the case where those two operations are different.

If we distinguish the two then, I believe the performance increase won't cause problems.