httpapi: Added "last beat delta"
httpapi: Added "last beat delta"

The client doesn't know the exact timestamp of the server, so
we have to calculate this server-side.

walmartlabs/lovebeat,boivie/lovebeat,magnusbaeck/lovebeat,walmartlabs/lovebeat,boivie/lovebeat,magnusbaeck/lovebeat,walmartlabs/lovebeat,boivie/lovebeat,walmartlabs/lovebeat,magnusbaeck/lovebeat,boivie/lovebeat