BUG-582: use shared thread-safe SimpleDateFormat

This concentrates the per-parser DateTimeFormat into the pre-existing
yang-common one, lowering resource parser resource usage, while
remaining thread-safe.

Change-Id: I30d677ba4c66196bb5df551349c1d8abc31702b9
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-582: use shared thread-safe SimpleDateFormat"