Expose async transaction support in the C API

[nicola-cab]

What, How & Why?

Expose realm support for async transactions in the C API.
Fixes: #5771

☑️ ToDos

• 📝 Changelog update
• 🚦 Tests (or not relevant)
• C-API, if public C++ API changed.

[fealebenpae]

I think we need to change the names. In the C++ Realm class we talk about transactions (begin_transaction, cancel_transaction), but in the C API we talk about writes (realm_begin_write, realm_commit, realm_rollback). I suggest that for consistency's sake we use realm_async_begin_write, realm_async_commit, and realm_async_cancel, or something else that follows the existing terminology in the C API.

[nirinchev]

A few more questions from me.

[nicola-cab]

@fealebenpae all your comments have been addressed, @nirinchev, if you think that passing the realm in the callback is superfluous, let me know, and I will remove the parameter.

[nirinchev]

I'm just not sure why we'd need the realm instance in the callback. Supposedly, the caller already has an instance and they can capture that and use in the callback. It's possible I'm missing something though, so just wanted to check if that's the case.

[nicola-cab]

I'm just not sure why we'd need the realm instance in the callback. Supposedly, the caller already has an instance and they can capture that and use in the callback. It's possible I'm missing something though, so just wanted to check if that's the case.

No, I think you are right, let me remove it.

[nicola-cab]

@nirinchev I am about to merge this, are you OK with the changes?