Allow executemany to return rows

[LeonardBesson]

Fixes https://github.com/MagicStack/asyncpg/issues/448

This is a proposed change to support executemany with INSERT ... RETURNING.

The PR adds a flag parameter to executemany, but adding a seperate method like fetchmany was also mentioned.

@elprans would you consider something along those lines? I think the value here is allowing to take advantage of the query pipelining from executemany while avoiding the limitations of the proposed workarounds (like DEFAULT as mentioned in your stackoverflow post)