New ABI messing around

[rm5248]

First off: this does not currently compile.

This PR is really for a discussion to talk more about #519. To that end, this PR does a few things as a discussion point:

• Making APR optional. This makes it possible to use log4cxx using only the standard C++ library, at the risk of being less useful
• Refactoring log4cxx_time_t to be std::chrono::time_point instead. This results in better code semantics at the risk of making the code more annoying to deal with(chrono is not a very fun API)
• Removal of APR pool. Most of the time the pool argument to functions is not used, so this is a pretty straightforward removal. Anything that is using a pool for some reason should probably use something like a smart pointer instead
• Replacing the File implementation with classes from std::filesystem or std::fstream as appropriate
• Replacing other APR calls with C++ standard calls when available

If we want to do some/all of these, I would create some new PRs to do these in smaller batches, as this has become very time consuming to try and do since these are rather invasive changes.