logging: add XLOG() and XLOGF() logging macros

logging: add XLOG() and XLOGF() logging macros

Summary:
This adds new `XLOG()` and `XLOGF()` macros to the logging library.  These are
similar to `FB_LOG()` and `FB_LOGF()`, but do not require a Logger argument.
Instead, the log category is picked automatically based on the current file
name.  The basic algorithm for picking the category name is to replace
directory separators with '.', and to strip off the filename extension.

For instance, all `XLOG()` statements in `src/foo/bar.cpp` will log to the
category `src.foo.bar`.  This also works correctly in header files: `XLOG()`
statements in `src/foo/mylib.h` will log to `src.foo.mylib`

This should generally result in a good log category hierarchy without the user
having to spend additional time picking category names--we simply re-use the
decisions that they already made for their directory layout.

In general I expect the `XLOG()` macros to be convenient enough that users will
use `XLOG()` in almost all cases rather than using `FB_LOG()`.

The log category name used by `XLOG()` statements can be overridden using
`XLOG_SET_CATEGORY()`, but this only works in .cpp files, not in header files.

Reviewed By: wez

Differential Revision: D4920257

fbshipit-source-id: 7ffafd9a4c87e6fb5eb35d86e0eb86ef1ed5be95