Util: New getEstimatedDowntime() method

core/java/src/net/i2p/I2PAppContext.java

router/java/src/net/i2p/router/Router.java

router/java/src/net/i2p/router/RouterContext.java

router/java/src/net/i2p/router/tasks/Restarter.java

...java/src/net/i2p/router/transport/ntcp/NTCPTransport.java

router/java/src/net/i2p/router/util/EventLog.java