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
```

6 files changed, 6 insertions, 0 deletions

diff --git a/toxav/audio.c b/toxav/audio.c
index 75d3d477..6889ef09 100644
--- a/toxav/audio.c
+++ b/toxav/audio.c
@@ -29,6 +29,7 @@
 #include "rtp.h"
 
 #include "../toxcore/logger.h"
+#include "../toxcore/mono_time.h"
 
 static struct JitterBuffer *jbuf_new(uint32_t capacity);
 static void jbuf_clear(struct JitterBuffer *q);
diff --git a/toxav/bwcontroller.c b/toxav/bwcontroller.c
index 2d7dd659..05fac704 100644
--- a/toxav/bwcontroller.c
+++ b/toxav/bwcontroller.c
@@ -31,6 +31,7 @@
 #include "ring_buffer.h"
 
 #include "../toxcore/logger.h"
+#include "../toxcore/mono_time.h"
 #include "../toxcore/util.h"
 
 #define BWC_PACKET_ID 196
diff --git a/toxav/groupav.c b/toxav/groupav.c
index 8936801a..15fdd534 100644
--- a/toxav/groupav.c
+++ b/toxav/groupav.c
@@ -27,6 +27,7 @@
 #include <string.h>
 
 #include "../toxcore/logger.h"
+#include "../toxcore/mono_time.h"
 #include "../toxcore/util.h"
 
 #define GROUP_JBUF_SIZE 6
diff --git a/toxav/rtp.c b/toxav/rtp.c
index 1e178720..3d563a1f 100644
--- a/toxav/rtp.c
+++ b/toxav/rtp.c
@@ -32,6 +32,7 @@
 
 #include "../toxcore/Messenger.h"
 #include "../toxcore/logger.h"
+#include "../toxcore/mono_time.h"
 #include "../toxcore/util.h"
 
 enum {
diff --git a/toxav/toxav.c b/toxav/toxav.c
index 36bf9cc2..5abe1130 100644
--- a/toxav/toxav.c
+++ b/toxav/toxav.c
@@ -28,6 +28,7 @@
 
 #include "../toxcore/Messenger.h"
 #include "../toxcore/logger.h"
+#include "../toxcore/mono_time.h"
 #include "../toxcore/util.h"
 
 #include <assert.h>
diff --git a/toxav/video.c b/toxav/video.c
index 1126b8e6..5145c29c 100644
--- a/toxav/video.c
+++ b/toxav/video.c
@@ -32,6 +32,7 @@
 #include "rtp.h"
 
 #include "../toxcore/logger.h"
+#include "../toxcore/mono_time.h"
 #include "../toxcore/network.h"
 
 /**