Clean up JSON*Formatter.createText()

Clean up JSON*Formatter.createText()

Use proper try-with-resources blocks and take advantage of method
returns.

Also improve element ordering, with eventTime being emitted before the
body.

Finally we tie the constants used to model and highlight the problems
that are in the current implementation.

Change-Id: Ie8f8659833d91f87903d9936d8bf2c9fad5b96ff
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>