FIX: Exit server when connections are drained
FIX: Exit server when connections are drained

This allows seamless exits/restarts where already-connected clients
won't get any errors for in-flight requests.

Fix #550

tomateos/S3,lucisilv/Silviu_S3-Antidote,tomateos/S3,lucisilv/Silviu_S3-Antidote,lucisilv/Silviu_S3-Antidote,truststamp/S3,lucisilv/Silviu_S3-Antidote,scality/S3,scality/S3,tomateos/S3,lucisilv/Silviu_S3-Antidote,lucisilv/Silviu_S3-Antidote,lucisilv/Silviu_S3-Antidote,tomateos/S3,tomateos/S3,scality/S3,scality/S3,tomateos/S3,tomateos/S3,scality/S3,truststamp/S3,truststamp/S3,truststamp/S3,truststamp/S3,truststamp/S3,truststamp/S3,truststamp/S3,scality/S3,tomateos/S3,scality/S3,scality/S3,lucisilv/Silviu_S3-Antidote