resultset::async_read_some, resultset::async_read_all signature

[anarthal]

The current interface for resultset::async_read_some and resultset::async_read_all doesn't allow to reuse memory or custom allocators. This has been flagged during the review as a possible performance issue. Proposed solutions:

a) Change the function signature to async_read_all(vector<row, Allocator>&, CompletionToken&&), with the completion handler signature being void(error_code).
b) Remove the functionality altogether.
c) Change the function signature to async_read_all(RowOutputIterator, CompletionToken&&), with the completion handler signature being void(error_code).

If c) is chosen, then provide an example or a wrapper function allowing to read all the rows in a resultset into a std::vector.

[vinniefalco]

Oh I like the sound of RowOutputIterator