Psycopg 2.8 beta 2 released

> If no problem arise we will release the final package in a few days.

...and problems found us. The last ones were a few eventful days.

Things that happened between 2.8 beta 1 and 2:

- Fixed #856, which would have seen async/green connections to block
  in a scenario with multiple queries and interspersed notifies.
- 'pq_get_last_result()' dropped in order to fix the above; results
  are temporarily stored in the connection in order to survive multiple
  'poll()' calls, which turns out to be better for certain green paths
  (where e.g. a hidden cursor was created and destroyed just to hold
  a result temporarily).
- Fixed #829, which was causing connections to be closed by unrelated
  multiprocessing calls.
- Added 'connection.pgconn_ptr' and 'cursor.pgresult_ptr' to allow
  interaction between Python and libpq via ctypes or cffi, without
  waiting for a new major version or writing C code. I thought that this
  scenario was covered by the capsule object but it's not, and projects
  using psycopg have waited forever to have just a function of two
  exposed: this should make psycopg2 more flexible.

I feel like all the bugs of the world have been fixed. However, as the
async layer has changed so much, I'd like to ask for another round of
tests: you can install psycogp2 2.8 beta 2 with:

    pip install -i https://test.pypi.org/simple/ psycopg2==2.8b2

Cheers,

-- Daniele