Fabric: Bunch of small changes in ContextContainer

Summary:
So, changes:
* Correctness checks only in debug mode (codesize win?);
* `registerInstance` marked as const (because it's thread safe);
* ContextContainer::Shared also enforces constness;
* Using faster better::map;
* Using shared/RW mutex instead of regular one;
* SharedContextContainer got removed.

Reviewed By: sahrens

Differential Revision: D14920284

fbshipit-source-id: f0f8d970e7fae79a1abe3bc32827db9fd2d17e13