Validate use of std::forward when passing callback to transactions

The transactions lib utilizes couchbase::core::utils::movable_function for the callback passed into the couchbase::transaction::run methods. This change appears to result in a crash for the async transactions (acouchbase) in Python. The crash is due to the PyObject* callback and errback passed into the lambda used for the transactions callback are null when the lambda (i.e. callback) is executed.

Passing in the callback with std::forward helps to mitigate the crash, but it should be better understood if this is the appropriate path forward. So, creating this ticket to log the effort to better understand what is happening and if the current use of std::forward is the best approach.

Transactions references:

• transactions.hxx
• movable_function

Python references:
transaction's callback lambda