Add support for higher precision timestamps

[Lastique]

Switch Boost.Filesystem to use std::chrono/Boost.Chrono for various file timestamps. This would be an API breaking change, so there should be an option to revert to std::time_t timestamps (to be removed after some transition period). The built library must remain binary compatible with any user-selected representation of timestamps.

https://github.com/boostorg/log/issues/145 is an example where higher precision timestamps would be useful.