Switch time() calls to monotonic clock calls (#7597)

[libt2n] / codegen / TODO

- add option similar to gccs -MD (at the moment it is a fixed set of files generated and
  they are handled in the makefile snippet [codegen.make])
- it would really be much nicer if the client lib includes would not depend upon boost serialization
  i thought a solution would be to provide this optionally by wrapping the corresponding includes in a #ifdef
  but this does not work since command.hxx is included which depends on boost serialization headers anyway
  => we do not provide this for now
- naming scheme?!
  perhaps generated group class should not be prefixed by cmd_group_
- would be nice to reduce amount of changed files
  (codegen output is often the same as the existing files - something similar to ccache
  for codegen - but not overwriting output files if they are the same)