Fixes a few cases where use after free could happen, and one where it can be
easily reproduced.

When using the FSEvents back-end, both the `Watcher` and a `State` pointers are
passed as part of the context parameter to a C style callback.

Both of these pointers may be free-ed before the callback is called before this
commit.

After this commit, these two pointers, as well as all most others in the
codebase are replaced with `shared_ptr`.

This is a lazy fix and goes against C++ core guidelines - "F.7".

A secondary pass would use reference counting only in the shared ownership
cases. For most back-ends this shared ownership happens on: error types,
subscription maps/sets and state structs.