Rasdapy3 works poorly with postgres DB.

[apercov]

RELEASE compile, default_basedb postgresql
Every time I run systemtest of rasdapy3 different queries fail with some of the following errors:

Traceback (most recent call last):
  File "/home/apercov/rasdaman/systemtest/testcases_mandatory/test_rasdapy3/rasql.py", line 138, in execute
    res = self.query_executor.execute_read(self.validator.query)
  File "/home/apercov/rasdaman/applications/rasdapy3/rasdapy/query_executor.py", line 49, in execute_read
    res = self.ras_oqlquery.execute()
  File "/home/apercov/rasdaman/applications/rasdapy3/rasdapy/ras_oqlquery.py", line 90, in execute
    return self.execute_query(QueryType.SELECT)
  File "/home/apercov/rasdaman/applications/rasdapy3/rasdapy/ras_oqlquery.py", line 112, in execute_query
    }[query_type]()
  File "/home/apercov/rasdaman/applications/rasdapy3/rasdapy/cores/core.py", line 518, in execute_read
    self.query_str)
  File "/home/apercov/rasdaman/applications/rasdapy3/rasdapy/cores/remote_procedures.py", line 296, in rassrvr_execute_query
    _QUERY_TIMEOUT_SECONDS)
  File "/home/apercov/.local/lib/python3.6/site-packages/grpc/_channel.py", line 826, in __call__
    return _end_unary_response_blocking(state, call, False, None)
  File "/home/apercov/.local/lib/python3.6/site-packages/grpc/_channel.py", line 729, in _end_unary_response_blocking
    raise _InactiveRpcError(state)
grpc._channel._InactiveRpcError: <_InactiveRpcError of RPC that terminated with:
	status = StatusCode.UNAVAILABLE
	details = "Socket closed"
	debug_error_string = "{"created":"@1587735467.023400664","description":"Error received from peer ipv4:127.0.0.1:7002","file":"src/core/lib/surface/call.cc","file_line":1056,"grpc_message":"Socket closed","grpc_status":14}"
>

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/apercov/rasdaman/systemtest/testcases_mandatory/test_rasdapy3/rasql.py", line 270, in <module>
    main.execute()
  File "/home/apercov/rasdaman/systemtest/testcases_mandatory/test_rasdapy3/rasql.py", line 167, in execute
    self.db_connector.close()
  File "/home/apercov/rasdaman/applications/rasdapy3/rasdapy/db_connector.py", line 58, in close
    self.db.close()
  File "/home/apercov/rasdaman/applications/rasdapy3/rasdapy/cores/core.py", line 202, in close
    rassrvr_close_db(self.stub, self.connection.session.clientId)
  File "/home/apercov/rasdaman/applications/rasdapy3/rasdapy/cores/remote_procedures.py", line 102, in rassrvr_close_db
    _TIMEOUT_SECONDS)
  File "/home/apercov/.local/lib/python3.6/site-packages/grpc/_channel.py", line 826, in __call__
    return _end_unary_response_blocking(state, call, False, None)
  File "/home/apercov/.local/lib/python3.6/site-packages/grpc/_channel.py", line 729, in _end_unary_response_blocking
    raise _InactiveRpcError(state)
grpc._channel._InactiveRpcError: <_InactiveRpcError of RPC that terminated with:
	status = StatusCode.UNAVAILABLE
	details = "failed to connect to all addresses"
	debug_error_string = "{"created":"@1587735467.024076720","description":"Failed to pick subchannel","file":"src/core/ext/filters/client_channel/client_channel.cc","file_line":3941,"referenced_errors":[{"created":"@1587735467.024071507","description":"failed to connect to all addresses","file":"src/core/ext/filters/client_channel/lb_policy/pick_first/pick_first.cc","file_line":393,"grpc_status":14}]}"
>

Same behaviour can be noticed when running rasql.py queries outside of the test. However, it is hard to provide some specific examples of queries that fail all the time.