add a separate config for controlling negative dns caching
don't overwrite pgbouncer user on startup if it already exists
don't load auth file twice at boot

the stat wasn't getting cached on the initial load, so immediately
after booting the janitor would notice it "changed", and reload it
fix launching new connections during maintenance

match up waiting connections with tested/used until we run out.
launches new connection if there are more waiting than could be
fulfilled by refreshing current connections. this situation can
happen if you have high churn on client connections (i.e. with
transaction pooling), such that there is *always* at least one
connection in sv_tested or sv_used.
Merge pull request #15 from ccutrer/dev/config_load_cleanup

Dev/config load cleanup
Merge remote-tracking branch 'ccutrer/dev/latency_fix'
Merge remote-tracking branch 'ccutrer/dev/nxdomain_ttl'