stats: Store token rx and tx timestamps as 64-bit

Token rx and tx timestamps were computed and stored as 32-bit unsigned
integer but substracted in other parts of code from 64-bit integer.
Result was, that node with uptime larger than 49.71 days
(2^32/(1000*60*60*24)) reported wrong numbers for
stats.srp.time_since_token_last_received and in log message during long
pause (function timer_function_orf_token_warning).

Solution is to store rx and tx data as 64-bit integer.

Fixes #761

Signed-off-by: Jan Friesse <jfriesse@redhat.com>
Reviewed-by: Christine Caulfield <ccaulfie@redhat.com>