Perf: Improve speed of uuid conversion

This patch adds a benchmark class measuring the various uuid conversions
done within Flow.  These code paths are important as we do multiple
id conversions (postId, revId, lastEditId, etc) per revision output
over the api.

This shows the method for converting values to timestamp was
inefficient. Most conversions are locally measuring sub .1ms per
operation, but hex to timestamp was taking 4x longer than the
next longest operations (alnum to hex and vice versa).

This patch adds a benchmark class measuring these conversions, and replaces
the slow hex2timestamp implementation with a more performant version. For
case 1-4 in the benchmark going over common use cases I see a 3x overall
improvement. This patch additionally adds a test asserting the extracted
timestamp is valid.

Change-Id: Ib933b750845999511bd7eec7c823c2539393d60e
Merge "Perf: Improve speed of uuid conversion"