appender: impl MakeWriter for RollingFileAppender

[hawkw]

Motivation

Currently, tracing-appender's RollingFileAppender does not implement
the MakeWriter trait. This means it can only be used by either
wrapping it in NonBlocking, or by wrapping it in a Mutex. However,
this shouldn't be strictly necessary, as &File implements io::Write.
It should thus only be necessary to introduce locking when we are in the
process of rotating the log file.

Solution

This branch adds a MakeWriter implementation for
RollingFileAppender. This is done by moving the file itself inside of
an RwLock, so that a read lock is acquired to write to the file. This
allows multiple threads to write to the file without contention. When
the file needs to be rolled, the rolling thread acquires the write lock
to replace the file. Acquiring the write lock is guarded by an atomic
CAS on the timestamp, so that only a single thread will try to roll the
file. This prevents other threads from immediately rolling the file
again when the write lock is released.

I...should probably write tests for that, though.

[davidbarsky]

LGTM