Fixed lag meter getting stuck.
Also lag_sent in struct server should have always been an unsigned long.

Fixes #749.
Now make_ping_time() in common/util.c returns the time in milliseconds instead
of microseconds.
In fe_set_lag(serv, lag) the lag argument is now in milliseconds as well.

Fixes #758.
Merge pull request #890 from orium/lagmeter-fixes

Lagmeter fixes