Critical-section in `sl_thd_alloc` and `sl_thd_free`

cFE unit-tests have thread deallocation tests and found out that thread was being woken up from timeout-queue when it is being deleted and also more subtle issues on deleted threads. To avoid races, and to cleanup the thread (except the actual system resource) use critical-section and delete it from timeout queue and scheduling policy module. This is what @Others and I've debugged recently.
It would be good if sl_thd_alloc is also atomic w.r.t sl library, so that there will not be races in backend allocations when users dynamically allocate more threads. Added that.
removed assert when there is nothing to schedule from sl_mod_schedule.
also added basic unit-tests for sl_thd_free. (idle thread runs after all test threads are deleted in the system.)

Reference: PR #262

Choose one (Mandatory):

[ ] This PR is for a code-review and is intended to get feedback, but not to be pulled yet.
[x] This PR is mature, and ready to be integrated into the repo.

@gparmer @Others

As part of this pull request, I've considered the following:

[x] Comments adhere to the Style Guide (SG)
[x] Spacing adhere's to the SG
[x] Naming adhere's to the SG
[x] All other aspects of the SG are adhered to, or exceptions are justified in this pull request

[x] I've made an attempt to remove all redundant code
[x] I've considered ways in which my changes might impact existing code, and cleaned it up
[x] I've formatted the code in an effort to make it easier to read (proper error handling, function use, etc...)
[x] I've commented appropriately where code is tricky
[x] I agree that there is no "throw-away" code, and that code in this PR is of high quality

I've tested the code using the following test programs (provide list here):

gwsystems / composite