This patch aims to:
* Unify connection & querying logic of ZenithPagerserver and Postgres.
* Mitigate changes to transaction machinery introduced in `psycopg2 >= 2.9`.
Now it's possible to acquire db connection using the corresponding
method:
```python
pg = postgres.create_start('main')
conn = pg.connect()
...
conn.close()
```
This pattern can be further improved with the help of `closing`:
```python
from contextlib import closing
pg = postgres.create_start('main')
with closing(pg.connect()) as conn:
...
```
All connections produced by this method will have autocommit
enabled by default.
There was no guarantee that the SELECT FOR KEY SHARE queries actually
run in parallel. With unlucky timing, one query might finish before
the next one starts, so that the server doesn't need to create a
multixact. I got a failure like that on the CI:
batch_others/test_multixact.py:56: in test_multixact
assert(int(next_multixact_id) > int(next_multixact_id_old))
E AssertionError: assert 1 > 1
E + where 1 = int('1')
E + and 1 = int('1')
This could be reproduced by adding a random sleep in the runQuery
function, to make each query run at different times.
To fix, keep the transactions open after running the queries, so that
they will surely be open concurrently. With that, we can run the
queries serially, and don't need the 'multiprocessing' module anymore.
Fixes https://github.com/zenithdb/zenith/issues/196
