Fixes https://emqx.atlassian.net/browse/EMQX-11771
For the same reasons as we inject `emqx_bridge_*` applications as dependencies to
`emqx_bridge` when starting the node. Already configured connectors are started when
`emqx_connector` application starts, and may lead to crashes and noise in the logs. One
example is to configure a mongodb bridge and restart the node.
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.
Serge Tupchii