Bug#29041740: libmytime: Create separate convenience library of my_time.cc and link mysys and other libraries/executables with it.
- Refactor time functions into my_time and my_systime.
- Re-implement functions with std::chrono where possible/desirable.
- Move the mysql_field_type enum to include/field_types.h and make it fwd-declarable in C++11 and up.

Change-Id: Iebca7c694827607bc18f04b1105988d7c7e4e7f4