text: Title: Feature request:  Support client side keepalives
  Body: <!-- Enter your issue details below this comment. -->
  
  I have been using version 0.20.1.  I was able to configure server side TCP keepalives via the server_settings dict object, but  I can't seem to find the place where I can set client side keepalives from either a connection pool or a connection object.  Does it make sense for an async library like asyncpg to support something like this?
  
  Thanks and Regards,
  Keith
  