Consolidate/Cleanup object_store code

[appden]

Right now we check for realm->is_in_transaction() inside object_accessor code for settings properties and creating new objects, but in the JSC layer for deleting objects and mutating arrays. All of this checks should be inside the object_store layer, which may mean adding new C++ methods for deletion and mutations. This will be good anyways in order to have less code duplication inside the Node API.

Also, from @alazier:

We will want to define a new exception type for mutations outside of write transactions, as other platforms will need to catch this and possibly handle this specific cases anywhere we throw from the shared object store code.

[alazier]

We need to cleanup our object store changes and get them merged to the object store repo. This includes:

• checking for transactions during mutations
• common path for property getters
• exception classes for common exceptions
• pull out ObjectArray into its own class/file