Modify `make-conn-pool` for common-use case: nil pool-opts now gives default pool
Modify `make-conn-pool` for common-use case: nil pool-opts now gives default pool

This previously gave a degenerate pool which is a pretty uncommon desire. The
`{}` conn idiom now gives more sensible behavior by default.

A degenerate pool can still be created by using `:none` for the pool opts.

ptaoussanis/carmine,tmcf/carmine,jackscott/carmine