Add tests to SingleThreadedSchedulerClient() and document the memory model

Summary:
This is a backport of Core PR13247

Add Unit Test for SingleThreadedSchedulerClient

Ensures ordering of callbacks within a SingleThreadedSchedulerClient
with respect to each other

Update documentation for SingleThreadedSchedulerClient() to specify the memory model

Update ValidationInterface() documentation to explicitly specify threading and memory model

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3169