Reduce pool size to 8 and 16
Reduce pool size to 8 and 16

Since Ecto 2.0 no longer lazily starts connections,
let's reduce the size to 8 for faster boot times in
dev and 16 in prod to avoid reaching limits.

Closes #1730.
mitchellhenke/phoenix,jwarwick/phoenix,stevedomin/phoenix,Joe-noh/phoenix,iStefo/phoenix,Ben-Evolently/phoenix,bsmr-erlang/phoenix,pap/phoenix,optikfluffel/phoenix,trarbr/phoenix,korobkov/phoenix,trarbr/phoenix,korobkov/phoenix,phoenixframework/phoenix,ybur-yug/phoenix,0x00evil/phoenix,jwarwick/phoenix,iStefo/phoenix,ephe-meral/phoenix,phoenixframework/phoenix,michalmuskala/phoenix,gjaldon/phoenix,nshafer/phoenix,ericmj/phoenix,michalmuskala/phoenix,rafbgarcia/phoenix,bsmr-erlang/phoenix,Kosmas/phoenix,dmarkow/phoenix,nshafer/phoenix,rafbgarcia/phoenix,mschae/phoenix,CultivateHQ/phoenix,mobileoverlord/phoenix,wsmoak/phoenix,ybur-yug/phoenix,ericmj/phoenix,cas27/phoenix,Gazler/phoenix,mobileoverlord/phoenix,pap/phoenix,CultivateHQ/phoenix,evax/phoenix,gjaldon/phoenix,keathley/phoenix,eksperimental/phoenix,stevedomin/phoenix,0x00evil/phoenix,keathley/phoenix,phoenixframework/phoenix,jamilabreu/phoenix,Gazler/phoenix,0x00evil/phoenix,dmarkow/phoenix,Joe-noh/phoenix,cas27/phoenix,rafbgarcia/phoenix,Kosmas/phoenix,evax/phoenix,eksperimental/phoenix,jamilabreu/phoenix,mitchellhenke/phoenix,ephe-meral/phoenix,optikfluffel/phoenix,mschae/phoenix,wsmoak/phoenix,nshafer/phoenix,optikfluffel/phoenix