SQL: More forgiving Avatica server.

[gianm]

Some changes to make life easier for users that aren't being completely scrupulous
with closing connections and statements. And also, for dealing with the realities that
sometimes clients crash and we need to clean up stale connections promptly or else
the server becomes unresponsive to new ones.

• Automatically close statements that are fully iterated or that have
  errors, to prevent dangling statements from causing clients to hit
  open statement limits.
• Empower client auto-reconnects by throwing NoSuchConnectionException
  when appropriate.
• Try to close empty connections when we hit the open connection limit,
  rather than failing the newly opened connection. Client
  auto-reconnections mean this shouldn't cause problems in practice.
• Improve concurrency of the server by making "connections" a
  concurrent map.
• Lower default connection timeout to PT5M from PT30M.

[fjy]

👍