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, 3 insertions, 0 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index d3266e2f..9fe1c3c5 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -171,6 +171,8 @@ set(toxcore_PKGCONFIG_REQUIRES ${toxcore_PKGCONFIG_REQUIRES} libsodium)
 set(toxcore_SOURCES ${toxcore_SOURCES}
   toxcore/logger.c
   toxcore/logger.h
+  toxcore/mono_time.c
+  toxcore/mono_time.h
   toxcore/network.c
   toxcore/network.h
   toxcore/state.c
@@ -343,6 +345,7 @@ include(CompileGTest)
 unit_test(toxav ring_buffer)
 unit_test(toxav rtp)
 unit_test(toxcore crypto_core)
+unit_test(toxcore mono_time)
 unit_test(toxcore util)
 
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