fix: inherit configuration in run_parallel

[BugenZhao]

In Runner, we create a new runner for each parallelism without reusing self.conn, which is not good. What's worse is that we don't inherit other fields from self, resulting in add_label not taking effect when running in parallel.

[xxchan]

OK-ish as a quick fix

[xxchan]

I don't understand the comment here. Where's the make_conn passed to new?

[xxchan]

Oh, do you mean self.conn?

[xxchan]

Ok, so it seems self.conn is used to CREATE DATABASE, but we need new conns for individual databases.

[BugenZhao]

The idea of conn_builder here has something common with make_conn passed to new, but they're not compatible and seem to be kind of redundant to me. For example,

• The first arg is a host randomly picked from the hosts, which is strange. If the callers want load balancing between hosts, they are allowed to capture a slice of hosts and do random picking in the body of make_conn or conn_builder.

• The procedures to establish a connection to the database in make_conn and conn_builder are likely to the same, except for targeting different databases. We should unify them by changing the definition of MakeConnection. Only using the one passed in new to create databases in parallel mode does not seem to be a good design.