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/other/DHT_bootstrap.c b/other/DHT_bootstrap.c
index f263b471..b71ab0b1 100644
--- a/other/DHT_bootstrap.c
+++ b/other/DHT_bootstrap.c
@@ -33,6 +33,7 @@
 #include "../toxcore/LAN_discovery.h"
 #include "../toxcore/friend_requests.h"
 #include "../toxcore/logger.h"
+#include "../toxcore/mono_time.h"
 #include "../toxcore/tox.h"
 #include "../toxcore/util.h"