multi-tenant: fix loader dead lock

multi-tenant: fix loader dead lock

A dead lock could occur at start up, where a loader thread would
get stuck on it's condition variable, while the main thread was
polling the loaders task results.

The vector to the dead lock is as follows:

main	                        loader
DetectEngineMultiTenantSetup
-DetectLoaderSetupLoadTenant
--DetectLoaderQueueTask
---lock loader
---add task
---unlock loader
	                        lock loader
	                        check/exec tasks
	                        unlock loader
---wake up threads
	                        lock ctrl mutx
	                        cond wait ctrl
	                        unlock ctrl
-DetectLoadersSync
--lock loader
--check tasks
--unlock loader

Between the main thread unlocking the loader and waking up the
threads, it is possible that the loader has already moved ahead
but not yet entered its conditional wait. The main thread sends
its condition signal, but since the loader isn't yet waiting on
it the signal is ignored. Then when the loader does enter its
conditional wait, the signal is not sent again.

This patch updates the logic to send signals much more often.
It also makes sure that the signal is sent under lock, as the
API requires.

Bug: #6766.