Connections obtained with Database#using_connection should not be auto released

[ggiraldez]

This code:

require "db"
require "pg"

DB.open("postgres://localhost/foo") do |db|
  db.using_connection do |conn|
    conn.exec("select now();")
    conn.exec("select now();")
    conn.exec("select now();")
  end
end

fails with a closed stream (IO::Error) exception on the third call to exec. From a quick code inspection, #exec always returns the connection to the pool, regardless of how it was obtained, but I'd expect the connection to be held in a checkout state when used inside the using_connection block.

crystal-db version: 0.3.3
crystal-pg version: 0.13.1