[SPARK-2261] Make event logger use a single file.

Currently the event logger uses a directory and several files to
describe an app's event log, all but one of which are empty. This
is not very HDFS-friendly, since creating lots of nodes in HDFS
(especially when they don't contain any data) is frowned upon due
to the node metadata being kept in the NameNode's memory.

Instead, add a header section to the event log file that contains metadata
needed to read the events. This metadata includes things like the Spark
version (for future code that may need it for backwards compatibility) and
the compression codec used for the event data.

With the new approach, aside from reducing the load on the NN, there's
also a lot less remote calls needed when reading the log directory.
It's kind of weird to see this in a variable. Can we have a method that does the flushing for us instead?