config service: convert backing database to use mongodb

This is a much more natural fit for the config service which is really a
key value store, and has been abusing SQL. This will let us expand the
config stuff later to support profiles and per-account config values.

I have left in the SQL code for now so config changes are still being
sent there in the event of catastrophic mongodb failure.

http-service/pom.xml

@@ -116,6 +116,11 @@
 				</exclusion>
 			</exclusions>
 		</dependency>
+		<dependency>
+			<groupId>org.mongodb</groupId>
+			<artifactId>mongodb-driver-sync</artifactId>
+			<version>3.10.1</version>
+		</dependency>
 
 		<dependency>
 			<groupId>org.springframework.boot</groupId>