Keep track of certificate file timestamp and reload if it has changed

With the old method, it was possible for a certificate to remain used
eternally, and letsyncrypt.js can change it relatively often.
14 jobs for dailybuild_linux-x64 in 14 minutes and 33 seconds (queued for 21 seconds)
Status Job ID Name Coverage
  Build
passed #27170
FreeBSD
jsdoor-freebsd

00:03:53

passed #27171
Linux
jsdoor-linux

00:10:24

passed #27173
FreeBSD
jsdoor-windows

00:01:53

passed #27165
FreeBSD
sbbs-freebsd

00:05:27

passed #27160
Linux
sbbs-linux

00:14:17

passed #27163
Windows
sbbs-windows

00:05:13

passed #27172
FreeBSD
sexpots-freebsd

00:00:12

passed #27161
Linux
sexpots-linux

00:00:23

passed #27164
Windows
sexpots-windows

00:00:25

passed #27168
FreeBSD
syncdraw-freebsd

00:00:21

passed #27169
Linux
syncdraw-linux

00:01:28

passed #27166
FreeBSD
syncterm-freebsd

00:00:57

passed #27162
Linux
syncterm-linux

00:04:45

passed #27167
FreeBSD
syncterm-windows

00:01:42