change postConstruct functions from asynchronous to synchronous

[paul-marechal]

A problem in our code base is that some of those postConstruct functions return promises. Inversify won't wait on them, as the instance creation step assumes everything happens synchronously. This means that more often than not, by the time our asynchronously-post-constructed services are injected, dependents might try to call into uninitialized properties...

We can handle asynchronous processes in post-constructors, but it needs to be done with more care.

[msujew]

This was done with #12425.