Factor out time keeping code into its own module: mono_time.c.

It turns out, `unix_time` is also monotonic, and is used as such, so I've
renamed the new functions to `mono_time_*`.

2018-07-08:
```
00:01 <@irungentoo> the idea used to be that the unix_time() function
  could go backward in time but I think I might have started using it like
  if it could not after I changed it so that it would never go back in time
```

1 files changed, 1 insertions, 0 deletions

diff --git a/toxcore/DHT.c b/toxcore/DHT.c
index fe3c1159..2572b28a 100644
--- a/toxcore/DHT.c
+++ b/toxcore/DHT.c
@@ -29,6 +29,7 @@
 
 #include "LAN_discovery.h"
 #include "logger.h"
+#include "mono_time.h"
 #include "network.h"
 #include "ping.h"
 #include "state.h"