Add move assignment operator for cursor_managed

[canepat]

Custom move assignment operator for cursor_managed is required to close the current handle before moving (see also erigontech/silkworm#575).

[erthink]

Hmm, I will fix this (by my self) promptly.

[canepat]

Sorry, I've missed that cursor explicitly manages the handle, so probably the correct solution is a bit more involved.

[erthink]

Please check out the d8431a3

[erthink]

Minor refine commit' comment by the 3c574fc.

[erthink]

@canepat, thank you for reporting.

[canepat]

These changes raise now compiler errors on code using mdbx in cases like this:

mdbx::txn tx = ...
mdbx::cursor_managed source{tx.open_cursor(...)};
...
source.close();
source = tx.open_cursor(...); // compiler error here

which compiled and worked as expected before (the leak was present just if close is not called). I don't think that your goal is prohibiting the cursor_managed move assignment or am I missing something?

Apart from compiler errors, using the inherited move assignment operator in cursor_managed class seems insufficient to me because the base class operator does not dispose the handle. Isn't the cursor_managed class supposed to manage the handle implicitly in all cases?

The change proposed in this PR doesn't break the case listed above and also closes the handle automatically when moving if close has not been called explicitly.

[canepat]

I've seen now your rework in 464886a and it looks good to me, it should solve my issues in previous comment.