Change to use the new multi-goroutine logplexc.Client

This removes the need for pg_logplexcollector to do its own POSTs, et
al.  Although some reporting (and testing) should be done around the
drop/reject statistics, it seems like all that can be neatly packaged
into logplexc.

Overall, ostensibly gained features:

  * Concurrency
  * Log dropping
  * Buffering
  * Time-based background flush trigger

Signed-off-by: Daniel Farina <daniel@heroku.com>