Refactor basic time usages

[marc-mabe]

This refactors the internal usages of the current time.

• Introduced zend_time.h to be used instead of system time.h
• Added zend_realtime_spec as a wrapper around clock_gettime, timespec_get, gettimeofday and time using the real/wall clock providing a timespec with up to nanosecond resolution
• Added zend_realtime_get as a replacement for time but optionally providing nanosecond
• Added zend_monotime_fallback either pointing to zend_hrtime or wrapping zend_realtime_spec
  • this can be used for time measurements (like timeout handling) where a monotonic timer is preferable but a fallback to real/wall-time without high resolution requirement is still better than nothing
• Added helper macros for working with timeval and timespec
• Replace direct usages of the system time API with zend_realtime_* / zend_monotime_fallback
• migrate to timespec for current time but keep timeval as it's needed for file/streams

As a result:

• The code is better readable because system differences are encapsulated
• zend_realtime_get / zend_realtime_spec and zend_monotime_fallback will be available for sure because it falls back clock_gettime -> timespec_get -> gettimeofday -> time
• Y2038 problem of WIN64 due to long of timeval.tv_sec should be fixed time() and friends have Y2038 problem on 64 Windows #17856
• Increased time resolution of microtime() / gettimeofday(true) without BC break

Even thought gettimeofday may not be available it wasn't checked everywhere before and compiling would have been failed already if missing. Availability should now be checked but as it falls back and it must have been available before I removed the conditions around microtime, gettimeofday and uniqid