logging: improve the AsyncFileWriterTest discard test

Summary:
This improves the test that exercises the AsyncFileWriter message discarding
logic.

Previously each writer thread wrote a fixed number of small messages.  This was
fairly slow, and wasn't always guaranteed to reliably produce discards.
Now each writer thread writes larger messages, which produce discards faster.
The test also automatically stops after 10 separate discard events, so that it
finishes faster (typically a few hundred milliseconds, rather than 5+ seconds).

This also updates the test to use XLOG() internally rather than using fprintf()
to print to stderr, now that the XLOG() diffs have landed.

Reviewed By: wez

Differential Revision: D5261059

fbshipit-source-id: 120224706fee36948ef76efbeb579ccc56400c51