Fixes https://emqx.atlassian.net/browse/EMQX-10408
From an old conversation with @kjellwinblad:
> There are 3 pool_sizes
> - The buffer workers pool size, just exposed here: https://github.com/emqx/emqx/pull/9742
> - The topology.pool_size, which controls the pool size for the poolboy_pool in Kjell's
diagram (on mongodb's side).
> - The pool_size from emqx_connector_mongo:mongo_fields that controls the ecpool pool
size (on EMQX's side).
> So we actually want to set topology.pool_size = 1 and hide it from users.
firest