Update mesh->loop.now right after select() returns.

Update mesh->loop.now right after select() returns.

It can be that select() waits for a few seconds before an event or
timeout arives. We didn't update mesh->loop.now before calling any of
the callback functions, which meant they could use a time a few seconds
in the past. In particular, the last_ping_time of connections could be
set to a value such that they would immediately be considered timed out.