fix thread locking for nested transaction

[bmatsuo]

Nested transaction need to know that they are nested so they don't call runtime.UnlockOSThread while their parent is pending.

There's a pretty big warning about thread safety on the godoc for BeginTxn. Maybe at some point the "BUG:" part is removed and this is just a feature of the Txn type. IDK. For now I think it should be considered a bug.

Note: I made the Cursor type aware that it was created from a nested transaction so that its Txn method can work properly (the transaction will not unlock the goroutine on Commit/Abort).

[bmatsuo]

@szferi ping. can you take a look at this?

[szferi]

Sorry I'm busy I will try to find some time to review it in this week.

[bmatsuo]

Ok :smile: Just wanted to make sure you saw it.