Retry on operation error when connecting to the database
When there is no pooled connection available, `.getconn()` tries to connect to the database.
This connection may raise an OperationalError like this:
```
Traceback (most recent call last):
File "/usr/local/lib/python37/dist-packages/trytond/wsgipy", line 117, in dispatch_request
return endpoint(request, **requestview_args)
File "/usr/local/lib/python37/dist-packages/trytond/protocols/dispatcherpy", line 47, in rpc
request, database_name, *requestrpc_params)
File "/usr/local/lib/python37/dist-packages/trytond/wsgipy", line 84, in auth_required
return wrapped(*args, **kwargs)
File "/usr/local/lib/python37/dist-packages/trytond/protocols/wrapperspy", line 181, in wrapper
return func(request, pool, *args, **kwargs)
File "/usr/local/lib/python37/dist-packages/trytond/protocols/dispatcherpy", line 172, in _dispatch
readonly=rpcreadonly) as transaction:
File "/usr/local/lib/python37/dist-packages/trytond/transactionpy", line 110, in start
autocommit=autocommit)
File "/usr/local/lib/python37/dist-packages/trytond/backend/postgresql/databasepy", line 263, in get_connection
conn = self_connpoolgetconn()
File "/psycopg2/poolpy", line 168, in getconn
return self_getconn(key)
File "/psycopg2/poolpy", line 92, in _getconn
return self_connect(key)
File "/psycopg2/poolpy", line 62, in _connect
conn = psycopg2connect(*self_args, **self_kwargs)
File "/psycopg2/__init__py", line 130, in connect
conn = _connect(dsn, connection_factory=connection_factory, **kwasync)
psycopg2OperationalError: server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request
```
issue